• Acta Photonica Sinica
  • Vol. 50, Issue 5, 95 (2021)
Fei YANG, Mengmeng LI, and Lanlan GAO
Author Affiliations
  • College of Science, Changchun University of Science and Technology, Changchun130022, China
  • show less
    DOI: 10.3788/gzxb20215005.0514004 Cite this Article
    Fei YANG, Mengmeng LI, Lanlan GAO. Single Longitudinal Mode 589 nm Laser in Composite-cavity[J]. Acta Photonica Sinica, 2021, 50(5): 95 Copy Citation Text show less

    Abstract

    A laser-diode pumped Nd:YVO4 and Nd:YAG single-longitudinal-mode 589 nm laser is presented. The fundamental waves of 1 064 nm and 1 319 nm simultaneously oscillate through an L type composite resonator. Through intracavity sum-frequency-generation in a KTP crystal (cut at type II phase matching),the single longitudinal mode 589 nm continuous wave is obtained by a birefringent filter which is consisted of a Brewster plate and the KTP crystal. The losses of S- and P- elements of the two fundamental waves are calculated by Jones matrix method. The losses of the first sub-longitudinal modes for 1 064 nm and 1 319 nm are 0.5% more and 2% more than their peak transmission modes respectively. Based on the above mentioned, a single-longitudinal-mode 589 nm laser is realized in experiment. The maximum output power is 58 mW, the amplitude fluctuation is less than 0.36%, and the line width is about 30 MHz. The results show that the birefringent filter technology is effective for single-longitudinal-mode double-wavelength oscillation and sum-frequency-generation lasers.

    0 Introduction

    Laser Diode (LD) pumped all-solid-state lasers have the advantages of high efficiency, long lifetime, compactness, and good liability1-3. The low-noise and Single-Longitudinal-Mode (SLM) 589 nm lasers are extensively required by biomedicine, sodium laser guiding star, city view and scientific research4-8. Due to the spatial hole-burning effect, the solid-state lasers generally operate in Multi-Longitudinal-Mode (MLM), and the coupling between the longitudinal modes during the nonlinear frequency conversion process will cause the irregular fluctuation in laser output power. In previous reports, there were several ways to realize the SLM9, laser generation such as the twisted mode cavity10, microchip cavity11, inserting etalon12, ring cavity13-14, birefringent filtering technology15, and so on. The SLM 589 nm lasers have been realized through the scheme of out-of-cavity Sum-Frequency Generation (SFG) and Raman doubling frequency16-20. The birefringence filter technology has been successfully used in frequency doubling lasers. However,few reports have been presented about this technology in SFG laser, which is mainly due to suppressing sub-longitudinal modes oscillating for two fundamental waves in a SFG laser is more difficult than that for one fundamental wave in a Second Harmonic Generation (SHG) laser.

    To obtain a highly stable, compact and narrow linewidth 589 nm laser, a composite-cavity laser with intracavity SFG is designed in this paper. The longitudinal-mode-selective mechanism of birefringence filter for double-wave oscillation is analyzed in theory. An SLM 589 nm laser is accomplished by birefringence filter in experiment. A Nd:YVO4 crystal is used to achieve 1 064 nm fundamental wave, and a Nd:YAG crystal is used to generate 1 319 nm fundamental wave. A linear cavity and an L type cavity are coupled by a flat-concave mirror. The birefringent filter composed of a Brewster Plate (BP) and a KTP crystal is placed in the overlap sub-cavity.

    1 Theoretical analysis

    The longitudinal-mode-selective theory of birefringent filter can be analyzed by solving the eigenvalue of the Jones matrix 21. The configuration of the SLM laser achieved by birefringence filter technology is shown in Fig.1. It is composed of a laser crystal, a BP and a KTP nonlinear crystal. The KTP crystal cut at type II critical phase matching is located near the output mirror M and acts as both SFG crystal and wave plate.

    Schematic diagram of birefringence filter

    Figure 1.Schematic diagram of birefringence filter

    By assuming that the Jones matrix of the laser crystal, KTP crystal and BP are W1W2 and P respectively, the Jones matrix of the fundamental wave in the cavity for one round trip can be expressed as

     M=W1×P×R-θ×W2×W2×Rθ×P×W1

    where, W1=100eδ1W2=100eδ2. δ1  δ2 are the relative phase delay of the laser crystal and the KTP crystal respectively; δi=2πλΔnd, in which λ is the wavelength of the fundamental wave of 1 064 nm or 1 319 nm,Δn is the refractive index difference of the fundamental waves in the crystals (laser crystal and KTP correspond to 0.2, 0.08 respectively), and d is the crystal length. P=q001, and q=2n/1+n22, in which n is the refractive index of BP at fundamental waves: n1064=1.473 n1319=1.472. Rθ=cosθsinθsinθcosθ is the coordinate conversion matrix between the laser crystal and the KTP crystal (laser crystal is S-P coordinate system, KTP is o-e coordinate system), and θ is the angle between sp and oe coordinate systems. The fundamental waves are linear polarized after passing through the BP. When the polarized direction of the incident fundamental wave coincides with the P  polarization plane of the BP, the losses suffering from the BP would be zero. These fundamental wavelengths are called peak transmission wavelength. The KTP crystal is placed at 45° on the horizontal plane. The optical axis of the KTP crystal is parallel to the crystal plane and the angle with the polarized direction of the fundamental waves is 45°, which isθ=45° . By solving the eigenequation of  MA=ζA  (where A is the ray matrix of incident fundamental wave), the eigenvalues of Mζ1,2 , can be obtained. The intensity transmission of the fundamental waves is given by the square of the modulus of the eigenvalue. The birefringence filter transmission at different fundamental wavelengths can be solved. The laser crystal is 3 mm long and the sum frequency crystal KTP is 7 mm long. The transmission curves of the S and P polarized fundamental waves are shown in Fig.2 and Fig.3 respectively. The Fig.2 shows the transmission curves of 1 064 nm fundamental wave when the BP is placed at an angle of 55.827°. The Fig.3 shows the transmission curves of 1 319 nm fundamental wave when the BP is placed at an angle of 55.310°. The upper convex curve is the transmission for the P polarized element, and the lower concave curve is the transmission for the S polarized element. It can be seen from the figures that the losses of the S polarized elements are larger (losses range from 57% ~ 67%) than those for P polarized elements (losses range from 0% ~ 15%). When the laser wavelengths are 1 063.63nm and 1 319.2 nm, the losses are zero for P polarized elements.

    1064 nm wavelength transmission as a function of wavelength

    Figure 2.1064 nm wavelength transmission as a function of wavelength

    1319 nm wavelength transmission as a function of wavelength

    Figure 3.1319 nm wavelength transmission as a function of wavelength

    Based on the total length of the resonant cavities (the linear cavity is 75 mm long and the L type cavity is 80 mm long), the contiguous longitudinal wavelength intervals of 1064 nm and 1319 nm are about 0.006 9 nm and 0.011 nm respectively. For the P polarized elements the losses of the first sub-longitudinal modes of both fundamental waves (as shown in Fig.2 and Fig.3) are 0.5% and 2% respectively. Therefore, if the losses difference is large enough the peak transmission fundamental wave mode would oscillate and the others will be suppressed and the SLM laser can be realized. According to the Fresnel formula, the refractive indexes of the quartz crystal BP for the two fundamental waves are: n1064=1.473  n1319=1.472. The Brewster angle are: θB1064=55.827°, θB1319=55.310°, and the angle difference is 0.517°. The tiny difference between these two fundamental waves makes it reasonable to realize SLM for two fundamental waves at the same time.

    2 Experimental study

    Fig.4 shows the schematic diagram of experimental setup. The laser diode (LD1) has a maximum output power of 4.68 W, and center wavelength of 808 nm. The pump power is focused by using the coupling system (OC1) into the center of the Nd:YVO4 crystal (3×3×3 mm3, Nd3+ doping concentration 1at.%). The Nd:YVO4 is wrapped in indium foil and mounted in a copper slot. The left flat side surface of the laser crystal is coated with High Transmission (HT) at 808 nm and high refection (HR) at 1 064 nm, acting as the input mirror for 1 064 nm. The right flat side surface is coated with Anti-Reflection (AR) at 1 064 nm. The maximum output power of the laser diode (LD2) is 2.34 W and the center wavelength is also 808 nm. The pump power is focused by using another coupling system (OC2) into the center of the Nd:YAG crystal (3×3×3 mm3, Nd3+ doping concentration 1at.%). The Nd:YAG crystal is wrapped in indium foil and mounted in a copper slot too. The incident flat side surface of laser crystal is coated with HT at 808 nm and HR at 1319 nm, acting as the input mirror for 1 319 nm. The output flat side is coated with AR at 1 319 nm. A flat-curvave mirror M3 with a curvature radius of 200 mm is placed in the cavity as a beam splitter. The concave surface is coated with HR (at 1 310~1 340 nm and 808 nm) and AR (at 650~670 nm and 1 064 nm). The planar surface is coated with AR at 650 nm~ 670 nm and 1064 nm. The output mirror M4 is a flat-concave mirror with a curvature radius of 100 mm. The concave surface is coated with HR at 1 319 nm/1 064 nm and HT at 589 nm. The flat surface is coated with HT at 589 nm. Both ends surface of the sum frequency crystal KTP (3×3×7 mm3) are coated with HT at 1 319 nm, 1 064 nm and 589 nm. The thermoelectric coolers (TEC1 and TEC2) are used to control the temperature of the LDs and the resonant cavity to ensure the stable operation of the laser. The BP (uncoated), with a thickness of 2 mm, is placed at a Brewster angle.

    Laser experimental device diagram

    Figure 4.Laser experimental device diagram

    First, the longitudinal mode conditions of 1 064 nm and 1 319 nm are measured with a Fabry-Perot (F-P) scanning interferometer (SA210-8B, 10G, Thorlabs). When there is no BP in the cavity, the 1 064 nm laser operates in multimode and 1 319 nm operates in less longitudinal mode, as shown in Fig.5(a) and (c) respectively. The reason is that the pump power for 1 319 nm is lower than that for 1 064 nm. When the BP is placed in the cavity and fine-tuned with angle, SLM oscillations of both 1 064 nm and 1 319 nm lasers are realized, as shown in Fig.5 (b) and (d). The line width of the 1 064 nm and 1 319 nm are 262 MHz and 240 MHz respectively. The experimental results illustrate that one BP method is effective for 1 064 nm and 1 319 nm to realize the SLM oscillation.

    The longitudinal mode of 1064 nm and 1319 nm

    Figure 5.The longitudinal mode of 1064 nm and 1319 nm

    Second, the longitudinal mode states at 589 nm are detected. The KTP crystal is placed near the output coupler. The position and angle of the KTP crystal are adjusted to obtain high conversion efficiency. It is found that the 589 nm laser is operating in multimode at all pumping energy level when the laser is oscillating without BP in the cavity. Fig.6 shows the F-P interferometer scanning spectrum of MLM 589 nm laser when the Nd:YVO4 is pumped with 2.1 W and Nd:YVG is pumped with 1.3 W.

    589 nm multimode with maximum pump power

    Figure 6.589 nm multimode with maximum pump power

    Third, the BP is put on the left side of the KTP crystal near the beam splitter. The position and angle of the BP is tuned until SLM exist at every Free Spectral Range (FSR) of the F-P analyzer and no any other sub-longitudinal modes appears. The results are shown in Fig.7(a). With an F-P interferometer (SA200-5B, Thorlabs) the spectral line-width of the 589 nm laser is measured. The FSR of the interferometer is 1.5 GHz with a minimum fineness of 200 and a finer resolution of 7.5 MHz. The spectral line-width of SLM 589 nm laser at the maximal output level is 30 MHz (As shown in Fig.7(b)). The line spectrum of SLM 589 nm is shown in Fig.8.

    Spectral analysis of 589nm SLM laser

    Figure 7.Spectral analysis of 589nm SLM laser

    589 nm laser spectrum

    Figure 8.589 nm laser spectrum

    When the incident pump power from LD2 remains at 2.3 W, the output power of MLM 589 nm and SLM 589 nm laser as a function of the LD1 incident pump power are shown in Fig.9. The maximum output power of MLM 589 nm and SLM 589 nm are 142 mW and 58 mW respectively when the LD1 power is 4.68 W. The possible reasons of that the output power of the SLM 589 nm laser is lower are as follows: First, there are extra insertion losses when the BP is placed in the cavity; Second, the pumping power of 1 319 nm is only 2.3W, which limits the sum-frequency efficiency.

    Output power of 589 nm as a function of the LD1 pump power

    Figure 9.Output power of 589 nm as a function of the LD1 pump power

    The stability experiment is conducted on an optical table in clean room at 25℃. The root-mean-square deviation is calculated to be 0.36% when the output power is at maximum, which is shown in Fig.10. The far-field spot of SLM 589 nm laser is measured, as shown in Fig.11, TEM00 single-transverse-mode operation is also confirmed.

    The amplitude fluctuation of SLM 589 nm output

    Figure 10.The amplitude fluctuation of SLM 589 nm output

    Diagram of SLM 589 nm laser far-field spot

    Figure 11.Diagram of SLM 589 nm laser far-field spot

    3 Conclusion

    We report an SLM 589 nm CW laser by using a birefringent filter consisted of single BP and KTP in the LD pumped composite-cavity. When the pumping powers of LDs are at maximum (1 064 nm resonator cavity with pump power at 4.68 W, 1 319 nm resonator cavity with pump power at 2.3 W), the SLM 589 nm yellow laser output power is 58 mW and the amplitude fluctuation is 0.36%. The line width of SLM 589 nm is 30 MHz. The experimental results show that longitudinal-mode-selective by a birefringent filter is an effective method for dual-wavelength oscillating laser. In fact, from Fig.2 and Fig.3 we can see that the larger the adjacent longitudinal mode interval is, the larger the additional loss the first sub-longitudinal mode suffers. By designing the resonator as short as possible, it will be easier to realize SLM SFG laser. Composite-cavity scheme is very helpful to realize mode matching and also it is useful to get the perfect energy ratio between the two fundamental waves. With the advantage of avoiding competition between two fundamental waves and longitudinal coupling, our scheme is promising to achieve stable high power SLM laser.

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    Fei YANG, Mengmeng LI, Lanlan GAO. Single Longitudinal Mode 589 nm Laser in Composite-cavity[J]. Acta Photonica Sinica, 2021, 50(5): 95
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