Design and Development of Intracavity Optical Parametric Oscillator-based Eye Safe Laser Operating at 20 Hz without Forced Air Cooling

In this paper we report the design and development of an electro-optically Q-switched diode pumped Nd:YAG laser with intracavity optical parametric oscillator, generating ~ 5 ns laser pulses of ~8 mJ energy at eye safe wavelength of 1534 nm. A Z-shaped laser resonator has been designed with porro prism end reflector in Q-switch arm containing RTP Q-Switch and a suitably oriented waveplate. The gain arm consists of a Ø3 x 72 mm Nd: YAG laser rod, pumped from one side by 3 x 5 bar laser diode array stack emitting total optical peak power of 740 W at 804 nm at 38 °C. Thermoelectric coolers (TECs) have been employed to maintain the optimum temperature of laser diode arrays and the combined heat load from the pump chamber and TECs is distributed over the system base plate with embedded heat pipes. Such cooling mechanism has eliminated the requirement of fins and fans in the laser system. Eye safe radiation is out-coupled through intra cavity KTA OPO (5 x 5 x 20 mm3) placed in the gain arm. Laser was operated at 20 Hz for several duty cycles of 10 min on and 10 min off and output energy remained stable within ±0.5 mJ without any forced air/liquid cooling.


Keywords:  Eye safe wavelengthintracavity optical parametric oscillatordiode pumped solid state laserelectro-optic Q-switching 


cSpeed of light
φ Photon density
nl Population inversion density
σStimulated emission cross-section of Nd: YAG
γ Inversion reduction factor
λ p , λ s , λ i Fundamental pump laser, signal, idler wavelength
l Length of the pumped region
tr Round-trip time
tc Cavity decay time
RPRS Output reflectivity at fundamental pump, signal wavelength
ω s , ω i Angular frequency of signal, idler
np,ns,ni Refractive indices of pump, signal and idler
τ 1064 nm Q-switched laser pulse width
γ s Ratio of backward to forward pump pulse
gsMode coupling coefficient of the cavity
deff Effective non-linear coefficient
Lo Optical length of OPO cavity
lo OPO crystal length
2aolo Round trip loss OPO crystal
ϵ Permittivity of free space

Lasers operating around 1540 nm are not only eye safe but have higher transparency in the atmosphere, silica based optical waveguides and fibers thus are in great demand for applications in laser range finding, remote sensing, lidar systems and optical communications etc. One of the most promising approaches for high-peak-power eye-safe laser sources is based on intracavity optical parametric oscillators (OPO)1-5. With the advent of high damage threshold nonlinear crystals and efficient diode-pumped Nd-doped lasers a resurgence of interest in intracavity OPO’s has been observed. Intracavity OPO has advantages of stability, compactness, low threshold and high efficiency.


Electro-optically Q-switched Nd: YAG - intracavity OPO eye safe lasers are generally based on mirror end reflectors, which are highly sensitive to misalignment. In this paper we report design and development of a Z-shaped laser resonator with porro prism end reflector in Q-switch arm. Besides providing mechanical stability the porro prism also results in better spatial profile of the output beam. Other special features of laser include solid state cooling mechanism employing thermoelectric coolers and heat pipes and use of motorized variable magnification (2-8X) beam expander at the output to control the laser spot size at different ranges. Present paper however focuses on the laser resonator design details and experimental results. Details pertaining to thermal management and beam divergence control shall be reported separately.

A Z-shaped laser resonator (Fig. 1) has been designed with porro prism end reflector in Q-switch arm containing RTP Q-Switch and a suitably oriented waveplate. Two high extinction ratio polarizers with high damage threshold coatings have been employed such that s-component of polarization oscillates in the resonator. The gain arm consists of a φ3 x 72 mm Nd: YAG laser rod, pumped from one side by 3 x 5 bar laser diode array (LDA) matrix stack emitting total optical peak power of 740 W at 804 nm at 38 °C. On opposite side of LDA, a gold coated V-shaped reflector has been used to reflect any unabsorbed pump radiation. Thermoelectric coolers (TECs) have been employed to maintain the optimum temperature of LDA and the combined heat load from the pump chamber and TECs is distributed over the system base plate with four embedded heat pipes. Such cooling mechanism has eliminated the requirement of fins and fans in the laser system. Eye safe radiation is out-coupled through intra cavity KTA-OPO (5 x 5 x 20 mm3) placed in the gain arm, which includes Nd: YAG laser rod pump chamber. Input face of the x-cut KTA crystal is coated with HT @ 1064 nm and HR @ 1534 nm while output face is coated with HR @ 1064 nm and 70 % R @ 1534 nm. A motorized variable magnification (2-8X) beam expander has also been employed at the output to control the laser spot size at different ranges.

3.1 Metallic package


For eye safe wavelength generation from Nd: YAG laser, intracavity OPO design philosophy has been chosen as it gives advantages of stability, compactness, low threshold and high efficiency. KTA crystal6-7 is preferred over KTP due to large nonlinear coefficient, high figure of merit, higher optical damage threshold and high transparency range (350-5300 nm).


3.1   Resonator without OPO

Laser resonator as shown in Figure 1 with a high reflecting mirror replacing OPO crystal has been analyzed theoretically for calculating the laser (1064 nm) parameters by solving the rate equations. The Q-switched laser (1064 nm) output parameters were analyzed numerically by solving the coupled rate equations of photon (φ) and population inversion density (nl) 8-9.


  { dϕ dt =ϕ[ 2σ n l l t r 1 t c ] d n l dt =γσcϕ n l             (1)

Theoretical estimations were carried out for the available input powers from LDA to validate the design capability for generating required energies and pulse widths at 1064 nm. Estimated temporal variation of inversion density and photon density is shown in Fig. 2 for the parameters listed in Table 1.

Figure 3 (a) and 3 (b) shows the effect of variation of input diode pump power (175 mJ, 260 mJ and 350 mJ) and output reflectivity on output energy and pulse widths respectively. Figure 4 shows the intracavity circulating energy in the resonator for carrying out intracavity operation. These results helped in selection of the diode peak power output for the generation of sufficient power at 1064 nm exceeding threshold intensity. Analyses of these results together with threshold calculations for OPO cavity as carried out in Section 3.2 are discussed together in Section 3.3.


Figure 1. Optical layout of laser resonator.



Figure 2. Temporal variation of inversion density and photon density .



Table 1. List of parameters.


3.2   Theoretical Estimation of Pump Laser Threshold Intensity for KTA OPO


KTA belongs to the orthorhombic crystal system, and therefore is optically biaxial. The indices of refraction for any propagation direction are given by the index ellipsoid defined by6


sin 2 θ cos 2 φ n 2 n x 2 + sin 2 θ sin 2 φ n 2 n y 2 + cos 2 θ n 2 n z 2 =0               (2)


where θ is the angle to the z axis, and φis the angle to the x axis in the x–y plane. The indices of refraction versus wavelength are given by Sellmeier equations6:


Figure 3.Variation of (a) output energy and (b) pulse width.


Figure 4. Intracavity circulating energy.


{ n x 2 =5.55552+ 0.04703 λ 2 0.04030 + 602.9734 λ 2 249.6806 n y 2 =5.70174+ 0.04837 λ 2 0.04706 + 647.9035 λ 2 254.7727 n z 2 =6.98362+ 0.06644 λ 2 0.05279 + 920.3789 λ 2 259.8645               (3)

Due to very small value of nonlinear coefficients in type I interactions, type II interactions are widely used in the KTA. For propagation in the x-z plane (φ = 0°), type II interactions correspond to type II phase matching in a positive uniaxial crystal (Non-critical phase matching, NCPM). For propagation in x-z plane, the polarization of the pump wave is along the y-axis (ordinary wave, ‘o-wave’) of the crystal as is that of the signal wave. The idler wave is polarized in the x–z plane (extraordinary wave, ‘e-wave’). For this type II phase matching, the wavelengths for three waves should satisfy energy conservation and the momentum-matching condition,


{ 1 λ p = 1 λ s + 1 λ i n y ( λ p ) λ p = n y ( λ s ) λ s + n( θ, λ i ) λ i                   (4)

Figure 5 shows the tuning curve calculated signal, idler wavelengths and effective non-linear coefficient for KTA optical parametric converter pumped at 1064 nm propagating in x-z plane. It shows that for NCPM condition, θ = 90°, φ = 0°, a signal wavelength of 1532.4 nm shall be generated with maximum nonlinear coefficient of 3.64 pm/V.The threshold pump intensity, Ith for a singly resonant NCPM OPO with reflection of the pump from the OPO output coupler is given by6:

I th = 1.8 κ g s l o 2 ( 1+ γ s ) 2 [ 25 L o cτ +2 α o l o +ln 1 R s +ln2 ] 2               (5) where κ= 2 ω s ω i d eff 2 n s n p n i ε o c 3               (6)


Figure 6 shows the threshold intensities required for the generation of KTA-OPO eye safe laser (signal) wavelength for different output reflectivities at 1534 nm.


Figure 5. Calculated tuning curves in x-z plane for KTA-OPO pumped at 1064 nm.




Figure 6. Calculated threshold intensity required for KTA-OPO eye-safe laser.


3.3   Results Analysis


Threshold intensity in Fig. 6 has been plotted using the pulse width of ~ 8 ns for the pump laser from Fig. 3 (b) (175 mJ diode output energy). As per thumb rule10 for designing an efficient OPO, pumping intensity should be at least 4 times above the threshold pump intensity for maximum efficiency. Therefore, for output reflector, Rs value of 70 %, the intensity required is ~ 75 MW/cm2 (4 times the value from Fig. 6, 16.2 MW/cm2). For the pump mode size of ~ 1.4 mm, the resultant intracavity power and circulating energy are 4 MW and 32 mJ respectively. From Fig. 4, intracavity circulating energy (for 175 mJ diode output energy) is ~ 31 mJ, while for the same threshold power requirements, if extra cavity operation were to be carried out, the diode output pump power would have to be increased by ~ 50% (Fig. 3 (a) for 40% R). Thus intracavity approach was finalized for efficient operation.

4.1   Temperature Stability


Before full assembly and alignment, temperature stability of LDA was observed by mounting only the pump chamber on the base plate with embedded heat pipes. Figure 7 shows the LDA case temperature variation with time, base plate just below the pump chamber and room temperature recorded using Yokogawa XL100 data logger. As can be seen in the graphs, before starting the operation, initial temperature of all points was at room temperature (~ 30 °C) the trend was recorded for ~ 7 min, then TEC controller was switched on with set temperature at 38 °C. Till the LDA power is switched on its temperature fluctuates within 2 °C but later a good temperature stability of ± 0.5 °C was achieved.It was observed that the plate temperature remains ~ 1 °C higher to room temperature after 10 minutes operation. Thus, numbers of cycles were repeated to see the long term stability. Figure 8 shows that there is no further rise in temperatures for 5 hrs continuous operation for 10 min ON / 10 min OFF duty cycle.


Figure 7. Temperature stability.


Figure 8. Temperature stability.



4.2   Free Running Performance


After resonator assembly and alignment, laser was first operated in free running mode (no voltage to Q-switch) by adjusting the waveplate to achieve maximum output through the polarizer in Q-switch arm (Fig. 9). In this case as the intracavity intensity does not overcome the threshold for nonlinear conversion, the OPO crystal acts like an HR mirror for fundamental wavelength. Figure 10 shows the free running performance at different input energies recorded using Ophir’s PE50BBDIFF detector with USBI-4 interface. As can be seen output energy remains stable and ~ 20 % optical to optical efficiency was achieved. The arrays were intentionally operated at ~804 nm to reduce thermal problems otherwise higher efficiencies are possible. In the present design, since the high reflectivity mirrors were coated directly on the OPO crystal, therefore Q-switched performance for 1064 nm was not carried out. However, free running energy results corroborate very well with the simulated Q-switched results (175 mJ input energy), where for 30 % output reflectivity, ~ 20 mJ Q-switched energy should be obtained (~ 80 % Q-switching efficiency). Figure 11 shows the uniform output spatial profile obtained for the free running operation.


Figure 9. Free running resonator setup.


Figure 10. Free running fundamental laser.


4.3   Eye Safe Operation


For eye safe operation, waveplate was adjusted to create high loss condition so that no oscillations occur in the resonator. Towards end of the pump pulse (245 µs duration) quarter wave voltage was applied to the Pockels cell and Q-switched output is achieved directly at eye safe wavelength. Figure 12 shows laser pulses for eye safe wavelength detected from gain arm and residual pump laser detected through Q-switch arm and recorded using New Focus 1623 and Alphalas UPD-200-UD detectors respectively. As can be seen clearly after OPO conversion pulse width reduces from ~ 7 ns to 5 ns. Laser was operated for several duty cycles of 10 min on and 10 min off at 175 mJ input energy, Figs. 13(a) and 13(b) depict output performance at 20 Hz. Energy output was stable within ± 0.5 mJ.



Figure 11. Free running spatial profile of 1064 nm output.



Figure 12. Pump and signal Q-switched laser pulses.


The far field divergence of the eye safe laser pulse was monitored using the Newport LBP beam profiler as shown in Fig. 14. The measured divergence using lens of focal length 100 mm was 1.9 mrad and 2.6 mrad in x and y directions respectively at 2X magnification of beam expander.

In this paper, theoretical design and experimental results of an electro-optically Q-switched diode-pumped Nd: YAG laser with intracavity KTA OPO, generating ~ 5 ns laser pulses of ~ 8mJ energy at eye safe wavelength of 1534 nm are reported. Advantage of intracavity operation has been highlighted using the theoretically generated curves for 1064 nm pump laser and threshold condition for OPO operation. The results match very well with the experimental results. Special features of laser include Z-shaped laser resonator with porro prism end reflector in Q-switch arm and solid state cooling mechanism employing thermoelectric coolers and heat pipes.


Figure 13. Signal laser at 20 Hz operation (a) 02 cycles, (b) 14 cycles.




Figure 14. Far field spatial profile of eye safe laser.


Far field beam divergence at 2 X magnification is measured to be 1.9 mrad and 2.6 mrad in x and y directions respectively.  Laser was operated at 20 Hz for several duty cycles of 10 min on and 10 min off and output energy remained stable within ± 0.5 mJ without any forced air / liquid cooling.

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Mr Atul Bhardwaj completed his BSc (H) Physics in 1996 and MSc (Physics) in 1998 from University of Delhi. He is working as Scientist at Laser Science & Technology Centre (LASTEC), Delhi. His current research interests include analytical and experimental studies on Q-switched diode pumped solid state lasers and nonlinear optical phenomenon.

Ms Lalita Agrawal completed her BSc (H) Physics and MSc (Physics) with electronics from University of Delhi in 1985 and 1987 respectively. Currently she is working as Scientist at LASTEC, Delhi and heading the Solid State Laser Group. She has more than 24 years of experience in the design and development of various types of solid state lasers.

Dr Anil K. Maini working as an Outstanding Scientist & Director of Laser Science & Technology Centre (LASTEC), Delhi. He has more than 35 years of R&D experience in diverse disciplines related to electronics, lasers and optoelectronics technologies and systems. He is Life Fellow of Institution of Electronics and Telecommunication Engineers (IETE) and a senior life member of Indian Laser Association (ILA). He has authored and co-authored twelve books. He has to his credit six patents (granted and filed) and more than 125 publications in National and International journals.