Laser light is not fully monochromatic so is it possible to reduce its spectral width without using any monochromator?
It depends on the 1) scale of laser bandwidth 2) your desire to keep the laser power 3) laser construction.
Semiconductor laser radiation is rather broadband and can be filtered by interference filter by several times. Fabri-Perot can be used for subsequent bandwidth compression/ Loss of power is proportional to bandwidth narrowing, .....Its a long story to discuss all cases
If you use Litman or Littrow configuration for tuning your semiconductor laser, the spectral width of your laser decreases.
Of course, if Sibasish introduces the wavelength selective element into the resonator of semiconductor laser, he may very narrow line with modest drop in power. However, Hamed, Sibasish still did not specified the type of his laser. I spoke about semiconductor laser only as example
Hi Sibasish, your question lacks too much details about the kind of laser (cw or pulsed, solid-state or semiconductor?) to call for an efficient answer. Apparently your laser is multimode: a preliminary work before implementing a line narrowing technique is to make it single-frequency (if the emission is cw) by insertion of a suitable intra-cavity solid etalon. Once done, the "monochromatic" spectrum fluctuations is mainly dominated by technical noise, such as cavity length acoustical perturbations that are deleterious to the spectral purity. There are several frequency stabilization techniques that can be implemented not only to frequency-stabilize the laser but also as a consequence to narrow its spectral width. A simple technique is called "side-of-fringe" locking technique during which one converts the frequency fluctuations into intensity fluctuations by positioning the laser frequency at the side of a high-finesse resonator that acts as a frequency discriminator, and then compare the transmitted fluctuating intensity to a stable reference voltage (the difference voltage acts as an error signal which, once integrated in time, can be fedback to a piezo transducer controlling the length of the laser cavity . A more performing line narrowing technique is the Pound-Drever-Hall technique, which not only stabilize the frequency but also the laser phase. Look at the speficic literature on laser frequency stabilization to get details on all available techniques.
What is the desired laser bandwidth? What type of laser is it? What is the spectral range of tuning?
Monochromator ,as a passive instrument, narrows the spectral width at the expense of optical loss.
In general, there are a number of intra-cavity set up using prism, littrow grating and grazing incidence grating and etalon FP that are assembled within the laser resonator including Hansch, Hanna and Litman (G-M and G-G) arrays.
In theory and practice you may achieve the spectral width as narrow as the single axial mode ~ few GHz.
Dear Duarte
You are right, however the the efficiency of various set up is different.
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