BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical communication module that is applied to a wavelength division multiplexing optical communication system. That is, the invention provides a stable optical system for locking a lasing wavelength of a laser source and a control system for the optical system. This optical system can be separately operated as a wavelength locker module but also can be used as integrated in the optical communication module having a laser source.  
           [0002]    The optical fiber communication features a long transmission length, high speed and large capacity, and strong immunity to electromagnetic noises, and hence a communication system that assures high reliability can be constructed. Formerly, in such systems, light of a single wavelength is transmitted on a single strand of optical fiber. However, in connection with the advent of large-capacity computerization societies in recent years, it is strongly demanded for the transmission capacity to be further increased. Therefore, there has been put to practical use the wavelength division multiplexing optical communication system where a plurality of optical signals each having a different wavelength are transmitted over a single strand of optical fiber, so that the number of communication channels is increased to achieve a larger capacity of the system. Normally, for the wavelength of light to be transmitted in an optical fiber, wavelength bands where the transmission loss of the optical fiber is low are used and such wavelength bands in a 1.3 μm range and in a 1.5 μm range are called windows of transmission. Since wavelength widths of these windows are limited, the narrower the wavelength spacing between adjacent channels becomes, the more the number of transmission channels can be increased. Presently, a frequency spacing are set to 200 GHz and 100 GHz, but there is a trend toward further narrower frequency spacing, such as 50 GHz and 25 GHz. Converting the above-mentioned frequency spacings into the wavelength spacings, those value become as narrow as approximately 1.6 nm, 0.8 nm, 0.4 nm, and 0.2 nm. When the wavelength spacing is narrowed to such levels, it becomes necessary for the wavelength of the laser source to be controlled to a constant value with pinpoint accuracy. This is because, if the wavelength of the laser source fluctuates to reach as far as the wavelength of the adjacent channel, there occurs crosstalk with the adjacent wavelength channel at a reception side and hence reliability of information transmission can not be assured. These wavelength (or, frequency) channels are called ITU-T (INTERNATIONAL TELECOMMUNICATION UNION-TELECOMMUNICATION STANDARDIZATION SECTOR) grids, and acknowledged widely as an ITU recommendation.  
           [0003]    Because of this background, there have been proposed several methods for controlling the wavelengths of the laser sources for the wavelength division multiplexing optical communication systems. For example, there has been devised a method for locking the wavelength of the laser diode by introducing a dielectric multi-layer filter, a Fabry-Perot etalon, or the like as a wavelength filter and giving feedback about the wavelength on an operating temperature of the laser diode. Among these wavelength filters, especially the etalon has a characteristics such that transmission peaks appear repeatedly in the wavelength according to the number of orders of multi-interference, and therefore, by adjusting the periods of a transmission curve to the ITU-T grids, a single wavelength filter can be used to lock a plurality of wavelength channels. For example, JP-A-79723/1998 discloses a method for locking the wavelength by dividing light having passed through the etalon into two portions, receiving the two portions with respective photo detectors, and subtracting the one from the other to derive a wavelength deviation signal, which will be used to lock the wavelength.  
         SUMMARY OF THE INVENTION  
         [0004]    It is the first problem of the present invention to stabilize the wavelength of the laser diode. Describing it in a concrete form, it is to achieve stabilization of a wavelength locker system in a semiconductor laser module (hereinafter referred to as a “laser diode module”) that has the said wavelength locking optical system in it.  
           [0005]    Among transmitted light beams of the etalon possessed by the wavelength locker system, any light beams that contribute to the wavelength deviation detection effectively are almost collimated light beams. Accordingly, light reflected from the etalon goes back to the laser diode via a converging lens, is reflected by a facet thereof, and is reflected again by the etalon, which is repeated to cause multiple reflections. Therefore, the reflected waves are multiple-interfered with one another, and consequently a distribution of the interference fringes varies in response to variation of the wavelength. Thus, light that arrives at a photo detector fluctuates to generate a ripple in the output, and there arises a problem of instability of this wavelength deviation signal.  
           [0006]    It is the second problem of the present invention to eliminate or alleviate the external feedback noise in the laser diode. That is, there is a drawback of generating so-called external feedback noise (returned light noise), namely fluctuation in a lasing intensity resulting from contention of the lasing mode of the laser diode itself with an external resonance mode that is generated by returned light to the laser diode being coupled to a waveguide thereof.  
           [0007]    A basic form of the present invention is a laser diode module that comprises at least: a laser diode device; a first detector element part for receiving, directly or indirectly, first light that is obtained when at least one of light emissions of said laser diode device is divided into two potion of light each traveling in a different direction; a second detector element part for receiving second light that is the other of the divided light beams at least via a wavelength selective member; and means of controlling the lasing wavelength of the above-mentioned laser diode device on the basis of outputs of the above-mentioned first and second photo detector parts, wherein a gap between the above-mentioned laser diode device and the above-mentioned wavelength selective member is formed so as to constitute an optical resonator with the multi-interference eliminated or alleviated therein.  
           [0008]    The followings are typical examples of constructing the optical cavity in which the multi-interference between the above-mentioned laser diode device and the above-mentioned wavelength selective member is eliminated. The first scheme is that polarization directions of the light emitted from the light source and of light returning to the said light source, as the returned light, are made different to each other. A more concrete example is as follows. The above-mentioned wavelength selective member in an optical path between the laser diode and the above-mentioned wavelength selective member itself is arranged so as to generate reflected light having a degree of polarization different from that of the incident light falling on the wavelength selective member. In the most preferable form, this difference between the directions of polarization is such that the directions of polarization are mutually orthogonal, which can ensure the most stable operation.  
           [0009]    The second scheme is reduction of mutual superposition of the following light beams: a light beam emitted from the light source; and a light beam that returns to the said light source as the returned light. For example, the angle of the reflection surface is tilted to avoid a superposition of the incident light and the reflected light. A straightforward configuration for this scheme is as follows. When the incident light is vertical to the surface, the surface is tilted with the respect to the optical axis of the incident light. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram of a wavelength control loop;  
         [0011]    [0011]FIG. 2 is a view showing an example of a relation between the lasing wavelength of a tunable laser diode and the temperature;  
         [0012]    [0012]FIG. 3 is a view showing an operation principle of the etalon according to the present invention;  
         [0013]    [0013]FIG. 4 is a diagram showing a wavelength spectral characteristic of the etalon according to the present invention;  
         [0014]    [0014]FIG. 5 is a view showing a configuration example of a wavelength locking optical system according to the present invention;  
         [0015]    [0015]FIG. 6 is a view showing a configuration example of the wavelength locking optical system according to the present invention;  
         [0016]    [0016]FIG. 7 is a view showing a configuration example of the wavelength locking optical system according to the present invention;  
         [0017]    [0017]FIG. 8 is a view showing a configuration example of the wavelength locking optical system according to the present invention;  
         [0018]    [0018]FIG. 9 is a view showing a configuration example of the wavelength locking optical system according to the present invention;  
         [0019]    [0019]FIG. 10 is a view showing a configuration example of the wavelength locking optical system according to the present invention;  
         [0020]    [0020]FIG. 11 is a view showing an example of a wavelength locked optical module according to the present invention;  
         [0021]    [0021]FIG. 12 is a view showing a configuration example of the wavelength locked optical module wherein tunable laser diodes whose wavelengths are locked by the wavelength locking optical system according to the present invention are arranged side by side to achieve enlargement of a tunable wavelength range;  
         [0022]    [0022]FIG. 13 is a plan view showing a sub-assembly to be used in the optical communication module; and  
         [0023]    [0023]FIG. 14 is a view showing an example of the optical communication module into which a wavelength locker part according to the present invention is built. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    As can be understood from the foregoing description, in order to reduce the multi-interference of the resonator that is formed between the facet of the laser diode and a wavelength deviation detection filter, a representative embodiment of the present invention adopts a configuration such that the facet of the laser diode and a wavelength deviation detection filter are arranged not in parallel to each other, or such that, by arranging an a wave plate inside the resonator, the polarization states of light beams are controlled to reduce superposition of the light beams having the same polarization, and further includes means for preventing the multiple reflection by arranging a polarizing element. Hereafter, several forms into which the present invention is carried out in practice will be described as examples.  
         [0025]    [0025]FIG. 1 is a configuration example to explain a control loop that acts as a base of wavelength locking according to the present invention. That is, the laser source  100  is a tunable laser diode device, for example, a DFB (distributed feed back) type or Fabry-Perot type laser diode device. Alternatively, it is a light source in which the DFB type laser diode device and an electro absorbing modulator are integrated. Alternatively, it is a light source in which a laser diode device equipped with a temperature control mechanism for tuning the wavelength and the electro absorbing modulator are integrated. That is, the tunable laser source  100 , a collimator lens  105 , abeam splitter  106 , a photo detector  110 , an etalon  108 , a photo detector  109 , a thermistor  403 , etc. are mounted on a thermoelectric cooling device  401  placed below a stem  300 , and the temperature of the laser  100  can be kept at a temperature in accordance with the resistance value of the thermistor  403  by a driving circuit  402  of the thermoelectric cooling device  401 . On the other hand, forward emitting light of the tunable laser source  100  is collimated by a collimator  101  and coupled to an optical fiber  104  via an optical isolator  102  and a converging lens  103  to travel therein for information transmission.  
         [0026]    This wavelength controlling loop has a structure in which, by switching a temperature control loop  488  that has been closed by a switch  405  to a wavelength control loop  489 , the wavelength control loop is switched. First, backward emitting light from the tunable laser source  100  is collimated by the collimator  105  and divided into transmitted light  107  and reflection light by the beam splitter  106  (a device for dividing light into two portions and guiding the two portions to different optical paths). The beam splitter  106  is of a shape formed by gluing two triangular prisms, or a thin parallel planar plate with a dielectric thin film deposited thereon, if a non-glued strucutre is favored. The reflected light undergoes photoelectric conversion to become an output monitor signal of the laser source  100 , which is compared with a previously-set value of the optical output and fed back to a driving circuit  404  to keep the output value of the laser source constant. Further, a configuration of the optical system for detecting wavelength deviation according to the present invention is as follows. Since, in the tunable laser source  100  like this, the lasing wavelength t varies as the temperature T changes as shown in FIG. 2, the lasing wavelength corresponding to the ITU-T grid can be selected by the operating temperature. In FIG. 2, the horizontal axis denotes the temperature of the laser diode and the vertical axis denotes the lasing wavelength. It is general application that the lasing wavelengths of the laser diode correspond to the wavelengths which are defined by the ITU-T grids. This matter will be described later. Incidentally, in the example of FIG. 2, the variation of the wavelength as a function of the temperature is 0.1 nm/° C. In this control loop of FIG. 1, the forward emitting light of the tunable laser source  100  is collimated by the collimator  101 , passes though the optical isolator  102  and a converging lens  103 , and is coupled to the optical fiber  104 . Thus, information is transmitted on the light.  
         [0027]    A configuration of an optical system for wavelength locking is as follows. That is, backward emitting light of the tunable laser source  100  is collimated by the collimator  105 , and divided into transmitted light  107  and reflection light by the beam splitter  106 . For the beam splitter  106 , there is one with a geometry where triangular prisms are glued together or a thin parallel planar plate with a dielectric thin film deposited thereon. If a non-glued strucutre is favored, the latter is of use. The transmitted light  107  further travels to the etalon  108 , and inside the etalon  108  light waves generated by the multiple reflections interfere with one another to create transmission peaks expressed by the following equation (1).  
           It= 1/(1 +F  sin 2 (ψ)),   (1)  
         [0028]    where  
           F= 4 R /(1 −R ) 2    (2)  
         ψ=(2 πnt /λ)cos θ′.   (3)  
         [0029]    As shown schematically in FIG. 3, the light incident on the etalon having a refractive index n at the point A 1  is divided here into reflected light R 1  and entered light. The entered light is further divided into light reflected at the point B 1  and light T 1  transmitted to the outside of the etalon. Reflected light generated at the point B 1  includes light to be emitted into outside of the etalon through the opposite surface thereof as light R 2  and light to be further reflected and emitted to the outside thereof as light T 2 . Thus, reflections are repeated inside the etalon to give the multiple reflections.  
         [0030]    [0030]FIG. 3 is expected to be seen regarding how to measure several quantities in the equations described above. Being shown in FIG. 3, R denotes surface reflectivity of either surface of the etalon  108 , n denotes the refractive index of the etalon, t denotes the thickness of the etalon, λ denotes the wavelength of light, and θ′ denotes an angle of light ray to the normal inside the etalon.  
         [0031]    Here, the Snell&#39;s law  
         sin θ=n sin θ′  (4)  
         [0032]    stands.  
         [0033]    In addition, a spacing of the multiple peaks in the transmitted light is called free spectral range (FSR) , which is expressed in the wavelength range by  
           FSR=λ   2 /2 nt    (5)  
         [0034]    and in the frequency range by  
           FSR=c /2 nt    (6)  
         [0035]    where c denotes the speed of light.  
         [0036]    Therefore, if the etalon is optimally designed with the thickness t, the refractive index n, and the surface reflectivity R, the FSR can be made to agree with the ITU-T grid spacing. Setting the FSR to the ITU-T grid makes it possible to realize the wavelength deviation detection with steep wavelength selectivity over a wide range of wavelengths. Thus, it is possible that a plurality of transmission ranges of light which exist with a predetermined wavelength spacing are made to exist and one of the transmission ranges is matched to a desired wavelength of the above-mentioned laser source.  
         [0037]    [0037]FIG. 4 is a view showing an example of the wavelength spectral characteristic of the etalon. In the figure, the horizontal axis represents the wavelength shift and the vertical axis represents the optical output. The transmission peaks in the etalon appear repetitively with increasing/decreasing wavelength as shown in FIG. 4. Further, the spacing of the transmission peaks is called, as mentioned above, free spectral range (FSR) and expressed by the equations (5), (6). Thus, transmitted light of the etalon so obtained is received by the photo detector  109  in FIG. 1 and converted into a photoelectric current. On the other hand, the reflected light from the beam splitter  106  is received by the photo detector  110  and converted into the photoelectric current.  
         [0038]    Then, the difference between photocurrent Pm flowing in the photo detector  110  for optical output monitoring, as described above, and the photo current Pt flowing in the photo detector  109  for wavelength monitoring, which is in obedience to the equation (1), that is arranged in the rear of the etalon  108  is assumed as the deviation signal A(λ).  
           A (λ)= Pt−Pm    (7)  
         [0039]    It is also appropriate that, for the deviation signal, a signal normalized by the light quantity  
           A (λ)=( Pt−Pm )/( Pt+Pm )   (8)  
         [0040]    is used. Thus, the wavelength deviation signal value A(λ), which was obtained with a high sensitivity, is fed back to the driving circuit  402  of the temperature control element  401  of the laser diode  100  so as to become zero or a constant value. It should be noted here that in order that the deviation signal represented by either equation (7) or (8) serves so that the wavelength captures one of the ITU-T grid wavelengths or its vicinity, the temperature of the laser diode is sensed from the value of the thermistor  403  beforehand and the operating temperature is set to a temperature corresponding to the said wavelength or its vicinity in accordance with the relation between the wavelength and the temperature obtained as shown in FIG. 2. After this preparation, the switch  405  in FIG. 1 is switched to the wavelength-locking loop. On the other hand, APC (AUTOMATIC POWER CONTROL) for keeping the power output of the laser diode constant compares, in the similar way, the output of the photo detector  110  with a set value and the difference thereof is fed back to the,driving current control circuit  404  of the laser diode  100 . The foregoing is for describing the control loop of wavelength locking.  
         [0041]    A ratio of the full width of the half maximum t of the transmission peak of the etalon to FSR is called finesse Φ, which has following relations of equations (9)-(11).  
           F= 4 R ·(1 −R ) z    (9)  
         Φ=(π{square root}{square root over ( F )})/2   (10)  
         Φ= FSR /ε  (11)  
         [0042]    Therefore, in order to achieve a steep spectral characteristic ε of the etalon, it is necessary to increase the reflectivities of the both sides of the etalon and also to cause reflection at the both sides to occur a number of times. Consequently, the angle of the incident light to the etalon needs to be set to the vertical angle (incident angle of zero degree) or its vicinity. If the incident angle deviates from the normal incidence, effective finesse decreases because the reflected light at each reflection constituting part of the multiple reflections is shifted in a direction perpendicular to the optical axis and the wavefronts of the respective reflections superpose only partially, hence reducing interference of light.  
         [0043]    However, if the incident angle of light to the etalon becomes close to the normal incidence, light resonates between a rear facet of the laser diode  100  and the etalon. This resonator can be analyzed by applying a method similar to that of the etalon that is a key component of the above-mentioned wavelength locker. Here, setting the reflectivity of the facet of the laser diode at 0.9, the transmissivity of the collimator lens at 0.95, the transmissivity of the beam splitter at 0.50, and also denoting the reflectivity of the etalon as Ir(λ), the effective reflectivity that governs the finesse of a newly formed Fabry-Perot resonator is given by  
           Re=SQRT (0.9×0.95×0.5× Ir ( A ))   (12)  
           Ir (λ)=1 −It.    (13)  
         [0044]    The finesse of the external Fabry-Perot resonator is given by putting Re in the equation (9) and (10). For example, if it is assumed Ir(λ)=0.5, then Re=0.546, which indicates the existence of a resonator with a finesse of approximately 4. Therefore, as the wavelength varies, ringing occurs in the output of the photo detector  110 . Since this phenomenon induces malfunction both in the APC (AUTOMATIC POWER CONTROL) of the laser diode and in the control loop of wavelength-locking, a countermeasure needs to be devised. This is a problem of the present invention.  
         [0045]    A first method of the countermeasure includes reduction in the reflectivity of the facet of the laser diode, and blocking the returned light onto the facet of the laser diode. Or, a direction of polarization of the returned light onto the facet of the laser diode is made orthogonal to that of the initial light to effect suppression of mutual interference. That is, a quarter-wave plate  111  is arranged between the collimator lens  105  and the etalon  108  as shown in F IG.  5 . Physical relationship of the quarter-wave plate  111  and the beam splitter  106  is such as is advantageous optically or in implementation.  
         [0046]    [0046]FIG. 5 is a view showing a configuration example of the wavelength locking optical system according to the present invention. This example is an example that uses the quarter-wave plate. Detail explanation of the example of FIG. 5 is omitted because the basic configuration other than the quarter-wave plate is the same as the example of FIG. 1.  
         [0047]    For a material of the wave plate  111 , normally, quartz, calcite, etc., all of which have optical anisotropy, are used. Since normally the light of the laser diode is almost linearly polarized in a direction of a waveguide stripe thereof (in a direction parallel to a face of the drawing), the light becomes a circular polarization after passing through the wave plate  111 , and after being reflected by the etalon  108 , passes through the wave plate  111  again backwards, and becomes a linearly-polarized light whose direction of polarization is vertical to the face of the drawing. Since these mutually orthogonal linearly polarized light beams do not interfere with each other, occurrence of the ringing which becomes a problem is reduced. However, light that has returned to the laser diode again and is reflected there returns to a linearly-polarized light whose direction of polarization is the same as that of the initial light by the transmission through the wave plate  111  and the reflection at the etalon  108 . In this particular case, if the loss of the resonator is sufficiently large, mutual interference is minute and the ringing can be ignored, but existence of such an expedient case depends on a degree of the loss of the resonator in comparison with an allowed value.  
         [0048]    [0048]FIG. 6 is an embodiment where the above-mentioned wave plate  111  is also served as the beam splitter  106  functionally. Actually, a member  112  of FIG. 6 is the above-mentioned member that serves basically as the quarter-wave plate and doubles also as the beam splitter. Regarding the example of FIG. 6, detail explanation is omitted because a basic configuration other than the member  112  is the same as the example of FIG. 1 or FIG. 5. That is, the normal beam splitter  106  is made of a vitreous material and hence optically isotropic. However, as in this example, the beam splitter can be provided with a function of being the quarter wave plate for light of oblique incidence if the beam splitter is made of an anisotropic material and the thickness thereof is designed to be equivalent to that of the said wave plate. Then, by designing a coating film on the surface, a beam splitter with a desired splitting ratio can be made up. If such a component is prepared, the number of the components can be reduced and such an effect as improves the degree of integration in a package can be brought out.  
         [0049]    [0049]FIG. 7 is a further embodiment according to the present invention. That is, the embodiment has a base configuration shown in FIG. 5 and FIG. 6 with the addition of a polarizer  113 . The polarizer  113  is arranged between the collimator lens  105  and the wave plate  111  or the wave plate  112  also serving as a beam splitter with its direction set to such a direction that linearly-polarized light of the laser diode can be transmitted. The linearly-polarized light having passed through the polarizer  113  passes through the wave plate, is reflected by the etalon, and again passes through the wave plate backwards to become a linearly-polarized light whose direction of polarization is orthogonal to that of the initial light (in a direction vertical to the face of the drawing). Since the linearly polarized light having a direction of polarization vertical to the face of the drawing is blocked by the polarizer  113 , it does not go back to the facet of the laser diode. Thus, the above-mentioned resonance is suppressed and the ringing in the output of the photo detector associated with the variation in the wavelength is reduced.  
         [0050]    [0050]FIG. 8 is for describing other means for similarly separating the returned light and the initial light through the use of polarization. That is, it is an embodiment wherein the beam splitter doubles as the polarizer. The linearly polarized light from the laser diode is made to enter a beam splitter  114 , as P (parallel) light, which transmits part of the light and reflects part of the light. This reflected light is received by the photo detector  110  and serves part of the output monitor/wavelength deviation detection signal. The transmitted light passes through the wave plate  111  to become circularly-polarized light, is reflected by the etalon  108 , again passes through the wavelength light  111  to become s (i.e., senkrecht) polarization, and enters the polarizing beam splitter  114 . In this example, the light is reflected there by a reflectivity of almost 100 percent, and thus the returned light to the laser diode is blocked. Needless to say, the transmitted light from the etalon  108  arrives at the photo detector  109  to give the wavelength deviation signal. Such an optical arrangement makes it possible to suppress the resonance occurring between the facet of the laser diode and the etalon, and also to close a stable wavelength control loop because malfunction in the wavelength deviation signal as well as in the optical output monitor are eliminated.  
         [0051]    [0051]FIG. 9 is a view showing a further embodiment according to the present invention. In this example, another stricture as the wavelength locking optical system is adopted. That is, a collimated light beam of the laser diode  100  is spatially divided into two portions of light whose boundary is an optical axis  201 , one of the two portions, as it is, being made to arrive at the photo detector  110  to form the optical output monitor. The other of the two portions passes through the etalon  108  to arrive at the photo detector  109  to give the wavelength deviation detection signal. Similarly, in the optical system like this, a resonator is formed between the facet of the laser diode and the etalon  108 , and the same problem of the invention as that described above may occur. To devise a countermeasure against the problem, the suppression of the resonance similar to that described above is performed with a combination of the polarizer  113  and the wave plate  111 , both of which are arranged in the optical path. The photo detectors  109  and  110  can be monolithically fabricated, and that, with the width of an isolation region ranging from a few μm to a few tens of μm.  
         [0052]    [0052]FIG. 10 is a view of another embodiment according to the present invention. This example is one that uses a slightly-tilted etalon arranged therein.  
         [0053]    In this example, a collimated light beam from the laser diode  100  is made to enter the etalon  108  after being diverged or converged to a certain degree. The photo detectors arranged just after the slightly-tilted etalon  108  are a split photo detector described in FIG. 9. The spectral characteristics of two light beams each passing through two parts of the slightly-tilted etalon  108  that correspond to split photo detecting areas of the split photo detector are much the same curve as depicted in FIG. 4 but different in the phase (the wavelength shift) to each other, and the subtraction of the both characteristics gives the wavelength deviation detection signal. Also in the wavelength locking optical system like this, there may occur the resonance between the etalon  108  and the facet of the laser diode, and hence a countermeasure against the resonance needs to be devised. For this purpose, this example performs polarization separation with a combination of the wave plate  111  and the polarizer  113  or only with the wave plate, and suppresses the resonance. In this case, the optical output monitor signal of the laser diode can be a sum of the signals of the photo detectors  110  and  109 .  
         [0054]    [0054]FIG. 11 is a plan view of an example of the laser diode light source  100  that is used in the present invention. In this example, a DFB type laser diode  300  and an electro absorbing modulator  301  are monolithically integrated. As shown in FIG. 1, the entire light source is mounted on a temperature controlling device such as the Peltier device, and hence the wavelength thereof can be tuned to a desired wavelength by using the relation between the wavelength and the temperature shown in FIG. 2. Note that, in tuning the wavelength, the temperature of the electro absorbing modulator  301  also changes accordingly, it is not likely that the characteristic of the modulator  301  is assured. Therefore, the present invention adopts a contrivance where a thin film heater  302  is formed adjacent to the optical modulator  301  to control the operating temperature of the optical modulator  301  always at an optimal value.  
         [0055]    [0055]FIG. 12 is another embodiment of the present invention. This example is an example that uses a laser diode device in which a plurality of laser parts are integrated.  
         [0056]    That is, DFB lasers  600  are integrated in the form of an array, optical outputs of the respective lasers are multiplexed into a single waveguide by a multiplexer  602  after passing through waveguides  601 , and the optical outputs are restored to a desired optical output by a semiconductor amplifier  603 , regaining losses of the optical outputs hitherto being experienced. Then, this light arrives at an exit facet  604  through the electro absorbing modulator  301  equipped with the thin film heater  302 . Subsequently, the light becomes a divergent light beam, is focused by the collimator lens  101 , passes through the optical isolator  102 , is transmitted through the beam splitter  106 , and is focused onto the optical fiber  104  with the converging lens  103  to travel therein for information transmission. Reflected light from the beam splitter  106  arrives at the photo detector  109  via the etalon or the dielectric multi-layer filter  108 . Here, a point that differentiates the present invention from the conventional configuration is that the optical isolator  102  is arranged between the beam splitter  106  and the collimator lens  105 . With such a configuration, the returned light from the etalon  108  by reflection is blocked by the optical isolator  102  and no resonator is formed externally; therefore no ringing occurs at the photo detector  109 . On the other hand, just behind the DFB lasers arranged are waveguide photo detectors  605 , which are for monitoring the respective outputs of the DFB lasers.  
         [0057]    [0057]FIG. 14 is a view showing a form example of a communication module package according to the present invention. FIG. 13 is a plan view showing an example of the sub-assembly that is to be used therefor. That is, this sub-assembly is such that the wavelength locking optical system according to the present invention is loaded in a communication module package  700  of a butterfly type with  14  pins. The etalon  108 , the wave plate  111 , the beam splitter  106 , the polarizer  113 , and the photo detectors  109 ,  110  are mounted on a substrate  702 , which is put into an optical path of the collimated light optical system of the laser diode that is operated at a different site and is lasing at the desired wavelength, and the incident angle of light ray to the etalon is adjusted and fixed to complete the sub-assembly. Following this, a sub-assembly  703  is loaded in the optical path of the collimated optical system for a desired wavelength that has been prepared in the package  700 . This method introduces an advantage of packaging that was made possible through the use of the collimated plane wave in the present invention.  
         [0058]    Making the wavelength locker part in the sub-assembly like this is hard to achieve by a method for wavelength-locking described in the above-mentioned JP-A-79723/1998. The reason for this is that, in order to adjust the angle of the divergent light beam to be entered into the etalon, a gap between the laser diode and the collimator lens needs to be fine tuned, and that, the angle of the tilted etalon needs to be fine tuned while monitoring the outputs from paired photo detectors, and consequently it is necessary to carry out adjustment in a narrow, work-in-process module. The embodiment of the wavelength locking optical system described above uses the etalon as the wavelength deviation detection filter by way of example, but in stead of the etalon, a dielectric multi-layer filter can be used. In this case, polarization controlling elements are employed to suppress the resonance between the dielectric filter and the facet of the laser diode. As described above in detail, adoption of the configuration composed of basic elements according to the present invention as a solution makes it possible to close a control loop of wavelength-locking that is stable optically and electrically and hence to get the performance of the etalon to the full. That is, the present invention achieves resolution of the technical problem hitherto impossible to solve, in terms of resolution of wavelength selection, utilization of light, increase in the density of packaging inside the laser source module, mechanical stability, correction of shift in locked wavelength due to the temperature change, etc. Further, with the utilization of the wavelength-controlling scheme according to the present invention, it becomes possible not only to lock the lasing wavelength of a laser diode but also to shift and then lock the wavelength thereof to any of the ITU-T grids. Therefore, according to the present invention, the laser source indispensable for a wavelength division multiplexing optical communication apparatus and a router device of wavelength channels can be provided.  
         [0059]    Hereafter, main forms into which the present invention is carried out in practice will be enumerated.  
         [0060]    A first form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, and there is provided means that guides the said divided light beams to a plurality of photo detectors, detects an amount of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents is kept at a constant value, a laser diode module and an application device in which the laser diode module is employed, characterized in that the laser diode device has means for reducing: mutual superposition between an initial light beam and a reflected light beam generated by a reflecting substance existing in the said light source and/or the optical path of the said focused light beam in terms of physical overlapping; or mutual superposition mentioned above in terms of polarization state.  
         [0061]    A second form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, and there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents is kept at a constant value, a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that a wave plate is arranged in the optical path of the said focused light beam.  
         [0062]    A third form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, and there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents is kept at a constant value, a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that a wave plate and a polarizer are arranged in the optical path of the said focused light beam.  
         [0063]    A fourth form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, and there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents is kept at a constant value, a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that the wave plate arranged in the optical path of the said focused light beam doubles as light beam dividing means.  
         [0064]    A fifth form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, and there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents equals a constant value, a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that a wave plate and a polarizing beam splitter are arranged in the optical path of the said focused light beam.  
         [0065]    A sixth form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, and there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents is kept at a constant value, a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that an optical isolator consisting of a polarizer and a Faraday rotator.  
         [0066]    A seventh form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents equals a constant value, and there is provided means that reduces mutual superposition between the initial light beam and a reflected light beam generated by a reflecting substance existing in the said light source and/or the optical path of the said focused light beam in terms of physical overlapping or mutual superposition described above in terms of polarization state, a laser diode module and an application device in which the laser diode module is employed, characterized in that the said wavelength filter is a Fabry-Perot etalon.  
         [0067]    An eighth form of the present invention is, in an optical module wherein a divergent light beam guided, directly or indirectly, from the rear or the front of a laser source or a light source in which the said tunable laser source and a modulator are integrated is focused with a lens, a wavelength filter is arranged in an optical path of the said focused light beam, there is provided optical path dividing means for dividing transmitted light or reflected light of the said wavelength filter, there is provided means that guides the said divided light beams to a plurality of photo detectors, detects a quantity of the lasing wavelength shift of the said laser source with the use of difference of photo currents flowing in the respective photo detectors, and controls the wavelength of the said laser source so that the said difference of the photo currents equals a constant value, and there is provided means that reduces mutual superposition between the initial light beam and a reflected light beam generated by a reflecting substance existing in the said light source and/or the optical path of the said focused light beam in terms of physical overlapping or mutual superposition described above in terms of polarization state, a laser diode module and an application device in which the laser diode module is employed, characterized in that the said wavelength filter is a dielectric multi-layer filter.  
         [0068]    A ninth form of the present invention is a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that a plurality of laser sources each having the said laser wavelength controlling means are arranged side by side and there is provided means that combines tunable wavelength ranges covered by respective laser sources.  
         [0069]    A tenth form of the present invention is a laser diode module and an application device in which the laser diode module is employed according to the above-mentioned first form, characterized in that part of or all of the said laser wavelength controlling means is built into a package of the laser source module.  
         [0070]    An eleventh form of the present invention is a laser diode module and an application device in which the laser diode module is employed, comprising at least: a laser diode device; a first photo detector part for receiving, directly or indirectly, first divided light that is a first portion of light when at least one of light emissions of the said laser diode device is divided into two portions of light each traveling a different optical path; a second photo detector for receiving said second divided light that is a second portion of light thus divided via at least a wavelength selective member; and means for controlling the lasing wavelength of the said laser diode device on the basis of outputs of the above-mentioned first and second photo detector parts, and characterized in that superposition between a beam of the light emission from the above-mentioned laser diode device and a beam of the reflected light from a reflecting substance in the optical path between the above-mentioned laser diode device and the above-mentioned wavelength selective member is smaller than superposition of the beams at a parallel reflection plane.  
         [0071]    According to the present invention, stabilization of the wavelength of the laser diode, namely, stabilization of a wavelength locking optical system in the laser diode module comprising the said wavelength locking optical system can be achieved.  
         [0072]    Further, according to the present invention, external feedback noise in the laser diode can be eliminated or alleviated.
The problem of the disclosed technology is that, in a wavelength locking optical system, a resonator is formed between a wavelength deviation detection device and a facet of a laser diode, which makes a control signal unstable. Resolution means for this is to arrange polarization controlling elements between a wavelength deviation detection filter and a laser diode to stabilize a deviation signal, and to perform wavelength--locking with the stabilized signal.