Frequency Stabilized Tunable Diode Laser And Its Application

Tunable external-cavity semiconductor laser has high efficiency and high reliability, its size can be only the size of the palm, its efficiency is about 30-40%, its continuous output power can be up to several watt, its price is very low and its life can be more than 2*104 h. External-cavity semiconductor laser also has excellent spectrum: high spectral purity (narrow spectral line width), tunable region of wavelength covered from the visible to the mid-infrared, the ability of generating ultrashort pulse (PS) output and high frequency (several GHZ) amplitude and frequency modulation. Therefore, the tunable external-cavity semiconductor laser has extensive application.The principle of tunable external-cavity semiconductor lasers can be explained as part of the output light of laser is feed back to the active-region, then the feedback light interacts with the light field of the active-region, therefore the multi-mode semiconductor lasers can be transferred to Single-mode, and the life of the photon is extended because of existence of the external cavity, as a result, the intensity-noise and phase-noise of the laser can be reduced and the linewidth of the laser be narrowed. External-cavity laser can be very easy to achieve linewidth below 1 MHz, improve the tolerance of external optical feedback and realize tuning of the laser frequency conveniently.Production and testing of the tunable diode laserTunable external-cavity semiconductor laser system designed by us uses Littrow configuration grating external-cavity semiconductor laser structure. The scheme is showed in Figure 1. figure1. scheme of Littrow configuration grating external-cavity diode laserLight from diode tune is collimated by a lens, then reflected by the reflection grating; Adjusting the reflection grating, the first-order reflection light feedback to the diode laser to generate single-mode and stable laser beam, zero-order reflection light provide laser output. We can use PZT to adjust the angle of the grating and the length of the cavity so that the different wavelengths of light oscillate back and forth in the external cavity, thus, generate different frequency,single-mode and stable laser beam.The mechanical components scheme of the tunable external-cavity semiconductor laser system designed by us is shown in Figure 2, on the whole, the semiconductor laser system is made up of diode laser, collimating lens, reflection grating, PZT and so on. They are installed on a base which has the good thermal conductivity and flexible substrates (such as aluminum, brass, etc.), we have to maintain constant temperature and current and change the grating angle by adjusting voltage to control laser wavelength. Figure 2 mechanical structure of tunable external cavity semiconductor laser (including 1.Base 2.Adjustment-bracket 3.Laser tube-bracket 4.Grating 5.Grating-bracket 6. Semiconductor refrigeration) Figure3. Scheme of tunable external cavity semiconductor laser interactingwith temperature control system and current control system Scheme of tunable external cavity semiconductor laser interacting with temperature control system and current control system show in Figure 3.We design the testing system showed in Figure 4. Light coming from the laser system pass through two Mirror and the first Beam Splitter to the Spectrometer to analyze spectra, pass through the second Beam Splitter to Wavelength Meter to measure the wavelength of the main mode, pass through F-P Etalon to measure linewidth. We test the different situation for diode laser with and without external-cavity, the results are very well.Figure 4 tunable external cavity semiconductor laser testing scheme Frequency stabilization of tunable diode laserWe stabilize frequency of tunable diode laser with the methods which don't need to modulate laser frequency directly, these methods can eliminate the additional frequency noise caused by direct modulation of the laser frequency and the operation is relatively simple, the core of these methods is to generate a frequency error signal. These methods generally can be divided into two categories, a large class of these methods use optical instruments as the reference frequency standards of laser frequency stability (such as Fabry - Perot cavity resonance frequency); Another large class use atomic and molecular transition frequency as the reference frequency, including DAVLL (dichroic atomic vapor laser lock) method and DFDL (Doppler-free dichroic lock) method. We use DAVLL, DFDL and modulation-free frequency stabilization of a laser based on a confocal F-P cavity to stabilize frequency of the tunable diode laser, then test and compare the effect of frequency stabilizationThe core of DAVLL is the generation of frequency error signal passing through zero at the lock frequency. Frequency stabilization using the Zeeman effect employs circular dichroism passing through an atomic vapor in the presence of a magnetic field. Linearly polarized light incident on the sample can be resolved into two counter-rotating circular componentsσ±. In the absence of the magnetic field, the optical resonance frequencies forσ±coincide. However, when a magnetic field B is applied, the Zeeman shifts lead to a difference between the resonance frequencies for the two circular polarizations of 2 gμB/, where g is the Lande factor andμis the Bohr magneton. After two beams of light passed through Rb atomic vapor, they are received by detectors respectively so that we can get a frequency error curve.DFDL combines Zeeman effect with saturated absorption. This frequency stabilization method eliminates the spectral Doppler-broadened so that diode laser can be locked on a narrow peak in the Doppler-free high-resolution saturated absorption spectrum of the atom and it has a broad range of signal. Experimental device show in Figure 5, plate 1,2 areλ/ 2 plates, plate 3 isλ/ 4 plate. Laser beam from the diode laser pass through OI, then it is divided into two beams, the weaker beam is used for frequency stabilization. This beam pass through a PBS and divided into two beams which are served as the strong pump beam and the weak probe beam. Pump beam are reflected by two mirrors, pass throughλ/ 4 plate, then incident on an atomic vapor in the presence of a magnetic field.σ+ andσ? polarized beams of the probe beam pass through Rb atomic vapor andλ/ 4 plate, then are separated by a PBS and detected by two photodiodes respectively. Subtraction and amplification of the signals from the two photodiodes by a DA produce the required dispersion-like DFDL signal and feedback this signal to the PZT of the diode laser to complete frequency stabilization. Figure 5. experimental schemeThe optical path and frequencies are different for axial and paraxial beams traveling through a confocal Fabry-Perot(CFP) cavity , thus the frequency difference between them which is a dispersion-like signal can be used to stabilize the laser f requency, this method is called modulation-free frequency stabilization of a laser based on a confocal F-P cavity. Experimental device show in Figure 6,the beam from the diode laser pass through OI, travel through a PBS to get two beams, one of which is used for frequency stabilization, then this beam become two beams by aλ/ 2 plate-PBS combination. After these two beams are reflected by 45°mirror, they pass through a confocal Fabry - Perot cavity with the different angle at the same time, then are received by photoelectric detectors to detect the frequency shift between two transmission curve. Figure 6. experimental scheme Application of tunable diode laser– lubricating oil contamination testingTesting oil contamination with tunable semiconductor lasers usually have two kinds of methods: light scattering law and theory of light . Theory of light using the intensity of transmission light test oil contamination , this method is relatively simple and easy to operate and the equipment made by this method easy to carry, furthermore, its price is relatively inexpensive, so we choose this method to test lubricating oil contamination. Figure 7 is the scheme of the testing device. The principle: two beams of light with equal intensity pass through two oil samples of the same brand, different contamination respectively. One is the new oil, the other is the old oil. After two beams of light passed through samples respectively, light intensity reduce to I1 and I2 and the detector receive the signal of I1 and I2, then calculate the signal ln( I 1 /I2)= k2d2-k1d1, the result of the calculation shows the contamination of lubricating oil.After we tested the lubricating oil of several cars and did some calculation, we got the application threshold of transmission rate under different reliability level.