detecting laser using solar cell and band pass filter
It seems you are confusing the laser light frequency with the signal it might be modulated with.

The system you describe is apparently modulating the lasers at 1 kHz.  This makes sense as normal things in the environment aren't going to produce light that comes and goes at a steady 1 kHz.  This also allows you to AC couple the light sensor signal.  That's useful since normal ambient light level can vary over a wide range.  Instead of looking for a certain light level, you look for the unique amplitude pattern that only your laser is producing.

The current produced by the light sensor is proportional to the overall instantaneous light intensity, up to probably a few 10s of kHz to 100s of kHz or more, depending on the photodiode.  After AC coupling this signal, you detect the presence of 1 kHz.  This can be done by a bandpass or tuned notch filter, or in firmware.

1 kHz is low enough that a narrow band filter at that frequency is quite readily doable in firmware.  Let's say you sample at 10 kHz.  That still gives you 100 &micro;s to process each sample, which is a "long" time for even a modest microcontroller or low end DSP.

One way to do this is to multiply the sample stream separately by 1 kHz sine and cosine.  Low pass filter each of these products separately.  The filter frequency will be the deviation from 1 kHz you can detect.  For example, if you low filter the two products to 20 Hz, then you will be filtering the original from 980 to 1020 Hz.

After low pass filtering the two products, square each, then add the squares.  The result is the square of the 1 kHz amplitude in your original signal.  If you're just detecting a certain level, then you can just as well do that with the square of the level than the level itself.