Chapter 1 Introduction
1.6 Overview of this dissertation
The accomplishments of this dissertation are the novel investigations in the realization of eye-safe lasers and the thermal management to improve the performance. The overall structures of the eye-safe lasers in this dissertation are summarized in Fig. 1.6-1. The organization of this dissertation is mainly as follows. Chapter 2 describes a high-pulse-energy eye safe laser up to ten mJ in the repetition rate of 1 ~ 20 Hz via optical parametric oscillator and the investigation of pulse dynamics under the thermally induce birefringence effect.
Chapter 3 shows an idea of diffusion-bond Raman crystal employed in the slef-SRS to improve the thermal effect and then the performance. Chapter 4 shows a passively Q-switched Er/Yb doped double cladding fiber laser with a novel AlGaInAs quantum well structure to be a saturable absorber. Chapter 5 describes a widely tunable laser accomplished with an OPO pumped by a passively Q-switched PCF laser with AlGaInAs quantum well structure to be a saturable absorber. Chapter 6 shows high-power AlGaInAs OPSLs with barrier pumping and in-well pumping and the investigations of thermal management via different pumping scheme.
Chapter 7, I list the contributions of this dissertation and the future work in the following researches.
Based on the gain mediums, there are three categories in laser types: solid-state lasers, fiber lasers, and vertical external cavity surface emitting laser (semiconductor disk laser).
Three pumping modes: continues-wave (CW), quasi-continuous-wave (QCW), and pulsed mode. The lasers are Q-switched in two ways to generate short pulse: passively and actively (Acoustic optical, AO). For the passively Q-switching, Cr4+:YAG and AlGaInAs QW/barrier are used as saturable absorblers. The overall structures of pulsed eye-safe laser reported in this dissertation are summarized in Fig. 1.6-1.
Chapter 1 Introduction
Fig. 1.6-1. The overall structures of eye-safe lasers reported in this dissertation.
Pulse
References
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Chapter 1 Introduction
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[15]. Makoto Yamada, Terutoshi Kanamori, Yasutake Ohishi, Makoto Shimizu, Yukio Terunuma, and Shoichi Sudo, “Pr3+-doped fluoride fiber amplifier module pumped by afiber coupled master oscillator/power amplifier laser diode,” IEEE Photon. Technol.
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Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
Chapter 2
Passively Q-switched Eye-safe Laser
with Optical Parametric Oscillator
2.1 Intracavity OPO Pumped with Nd-doped Laser
Intracavity optical parametric oscillator (OPO) is one of the most promising approaches for high-peak-power eye-safe laser sources since it can greatly reduce the lasing threshold due to the high pump intensity in the cavity. The advent of high damage threshold nonlinear crystals and diode-pumped Nd-doped lasers leads to a renaissance of interest in intracavity OPO’s. In recent years, a number of efficient eye-safe intracavity OPOs pumped by actively [1]-[3] or passively [4]-[6] Q-switched Nd-doped lasers have been demonstrated to produce pulse energies of tens of μJ with pulse peak powers of 1-100 kW.
The intracavity OPOs are mostly constructed with the coupled cavity configuration in which the resonators for the signal and fundamental wave fields are separate. Recently, it was found [7], [8] that the shared cavity configuration in which the pump and signal beams share the same resonator provides a substantially superior amplitude stability, in comparison with the coupled cavity configuration. Even so, the maximum peak power in a shared cavity is usually several times lower than that in a coupled cavity under the circumstance of the same output coupler. Therefore, it is a practical interest to explore an intracavity OPO in a shared resonator in quest of optimal pulse energies and peak powers.
In this chapter, I will report a ten-mJ eye-safe laser with an intracavity OPO pumped in a shared resonator. We first confirm that the threshold of an intracavity OPO pumped by a passively Q-switched laser is essentially determined by the bleach of the saturable absorber not by the signal output reflectivity. On the other hand, we numerically analyze and experimentally demonstrate the pulse behavior is affected by the birefringence induced from thermal effect in active medium.
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
2.2 Experimental setup
Figure 2.2-1 shows the experimental setup for an intracavity OPO pumped by a high-power quasi-continuous-wave (QCW) diode-pumped passively Q-switched Nd:YAG laser in a shared resonator. The fundamental laser cavity was formed by a coated Nd:YAG crystal and an output coupler. The OPO cavity entirely overlapped with the fundamental laser cavity. The pump source is a high-power QCW diode stack (Quantel Laser Diodes) that consists of three 10-mm-long diode bars generating 130 W per bar, for a total of 390 W at the central wavelength of 808 nm. The diode stack is designed with 0.4 mm spacing between the diode bars so the overall area of emission is approximately 10 mm (slow axis) × 0.8 mm (fast axis). The full divergence angles in the fast and slow axes are approximately 35° and 10°, respectively. A lens duct was exploited to couple the pump light from the diode stack into the laser crystal. The lens duct has the benefits of simple structure, high coupling efficiency, and unaffected by slight misalignment. The geometric parameters of a lens duct include r, L, H1, H2, and H3, where r is the radius of the input surface, L is the length of the duct, H1 is the width of the input surface, H2 is the width of the output surface, and H3 is the thickness of the duct [9], [10]. Here a lens duct with the parameters of r = 10 mm, L = 32 mm, H1 = 12 mm, H2 = 2.7 mm, and H3 = 2.7 mm was manufactured and used in the experiment. The coupling effect and the intensity distribution of intensity in exit surface of lens duct was simulated with commercial optical engineering software, the Advanced Systems Analysis Program (ASAP), as depicted in Fig. 2.2-2. The coupling efficiency of this lens duct was measured to be approximately 90 % and is in good agreement with simulation results.
Fig. 2.2-1. Experimental setup for an intracavity OPO pumped by a high-power QCW diode pumped passively Q-switched Nd:YAG laser in a shared resonator.
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
Fig. 2.2-2. (a) The schematics of diode power coupled by a lens duct. (b) ~ (d) The simulation result of intensity pattern in the image plane with 5-mm diameter from the exit surface of lens duct, 0.5 mm, 3 mm, and 5 mm respectively. The corresponding transmittance is 91.7 %, 87.9
%, and 68.4 %.
d=0.5 mm
d=3 mm d=5 mm
(a) (b)
(c) (d)
Image plane Lens duct
Diodes
d
The gain medium was a 1.0 at. % Nd:YAG crystal with a diameter of 5 mm and a length of 10 mm. The incident surface of the laser crystal was coated to be highly reflective at 1064 nm and 1573 nm (R>99.8%) and highly transmitted at the pump wavelength of 808 nm (T>90%). The other surface of the laser crystal was coated to be antireflective at 1064 nm and 1573 nm (R<0.2%). The nonlinear material for the intracavity OPO was an x-cut KTP crystal with a size of 4×4×20 mm3. The saturable absorber for the passive Q-switching was a Cr4+:YAG crystal with a thickness of 3 mm and an initial transmission of 60% at 1064 nm.
Both surfaces of the KTP and Cr4+:YAG crystals were coated for antireflection at 1573 nm and 1064 nm. All crystals were wrapped with indium foil and mounted in conductively cooled copper blocks. The output coupler had a dichroic coating that was highly reflective at 1064 nm (R > 99.8%) and partially reflective at 1573 nm. Several output couplers with different reflectivities (10% ≤ Rs ≤ 70%) at 1573 nm were used in the experiment to investigate the output optimization. The total cavity length was approximately 5.5 cm. The pulse temporal behavior at 1063 nm and 1571 nm was recorded by a LeCroy digital oscilloscope (Wavepro 7100; 10 G samples/sec; 1 GHz bandwidth) with a fast InGaAs photodiode. The spectral information was monitored by an optical spectrum analyzer (Advantest Q8381A) that employs a diffraction grating monochromator to for measure high speed light pulses with the resolution of 0.1 nm. In all investigations, the diode stack was driven to emit optical pulses 250 μs long, at a repetition rate less than 40 Hz, with a maximum duty cycle of 1%.
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
2.2.1 Theoretical analysis of threshold
The advantage of the intracavity OPO mainly consists in the exploit of high photon density of the fundamental wave. First of all, we analyze the maximum value of the intracavity photon density for the fundamental wave in a passively Q-switched laser. Next, we verify that the intracavity photon density of the present laser cavity can generally exceed the threshold of a singly resonant intracavity OPO by far, even though the reflectivity of the output mirror at the signal wavelength is nearly zero. In a passively Q-switched laser with a fast Q-switching condition, the maximum value of the intracavity photon density of the fundamental wave can be expressed as [11]
,max 1
the initial population density in the gain medium; σ is the stimulated emission cross section of the gain medium; lgm is the length of the gain medium; lcav is the cavity length; To is the initial transmission of the saturable absorber; σgs and σes are the ground-state and excited state absorption cross sections in the saturable absorber, respectively; R is the reflectivity of the output mirror at the fundamental wavelength; and L is the nonsaturable intracavity roundtrip loss. With the properties of the Nd:YAG and Cr4+:YAG crystals and the typical cavity parameters: σ= 2.8×10-19 cm2 , σgs = 8.7×10-19 cm2, σes = 2.2×10-19 cm2, lcav = 5.5 cm, R = 99.8%, To = 0.6, and L = 0.01, it can be found that φf,max can be up to 1.56×1017 cm-3.
With Brosnan and Byer’s equation [12], the threshold photon density for the double-pass pumped, single resonant OPO is derived to be given by
2
2 2
where gs is the mode coupling coefficient, γ is the ratio of backward to forward pump amplitude in the cavity; ω1, ω2 and ω3 are the signal, idler and pump frequencies, respectively;
n2 and n3 are the refractive indices at the signal, idler and pump wavelengths, respectively; τp
is the FWHM of the pump pulse; deff is the effective non-linear coefficient; ε0 is the vacuum permittivity; c is the speed of light; lnl is the length of the nonlinear crystal; Ls is the round-trip signal wave intensity loss in the cavity; and Rs is the output reflectivity at the signal wavelength.
Figure 2.2-3 depicts the calculated results for the dependence of the threshold photon density φf th, ( )Rs on the output reflectivity Rs with the properties of the KTP crystal and the 99.9% to 0.1%. As analyzed earlier, the obtainable intracavity photon density of the fundamental wave generally exceeds 1017 cm−3. Therefore, the intracavity OPO for any value of Rs can be promisingly generated in the shared cavity, as long as the pump energy can excite the fundamental wave to bleach the saturable absorber and to overcome the lasing threshold.
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
Fig. 2.2-3. Calculated results for the dependence of the threshold photon density on the output reflectivity Rs.
2.2.2 Experimental results and discussions
Figure 2.2-4 (a) shows the experimental results for the threshold pump energy versus the OPO output reflectivity. Experimental results confirm that the threshold pump energy is determined by the bleach of the saturable absorber not by the signal output reflectivity.
Consequently, a wide range of the signal output reflectivity can be used to optimize the output performance. Figure 2.2-4 (b) depicts the experimental results for the pulse energy of the signal output versus the signal output reflectivity. The optimal output reflectivity for the output pulse energy can be found to be within Rs =40~50%. With the optimum output coupler, the conversion efficiency from the diode input energy to the signal output energy is approximately 7%, which is slightly superior to the efficiency of 4~6% obtained in a coupled cavity [13].
Figure 2.2-5 (a)-(c) show the experimental results for the temporal shapes of the fundamental and the signal pulses obtained with three different output couplers. It can be seen that the pulse durations of the signal output are 4.4 ns, 2.1 ns, and 0.85 ns for Rs=60%, 50%, and 15%, respectively. The pulse width obtained with Rs = 15% is 2.4 times shorter than that obtained with Rs = 50%; however, the pulse energy is only 20% less than the maximum value.
In other words, the peak power reached with Rs = 15% can be nearly two times higher than that obtained with Rs = 50%. To be more accurate, the output peak was calculated with the experimental pulse energy and the numerical integration of the measured temporal pulse profile. Figure 2.2-6 depicts the experimental results for the peak power of the signal output versus the OPO output reflectivity. The optimal output reflectivity for the output peak power can be found to be within Rs =10−20%. With the optimum output coupler, the maximum peak power can be up to 1.5 MW.
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
Fig. 2.2-4. (a) Experimental results for the threshold pump energy versus the OPO output reflectivity. (b) Experimental results for the pulse energy of the signal output versus the OPO output reflectivity
(a)
(b)
Fig. 2.2-5. Experimental results for the temporal shapes of the fundamental and the signal pulses
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
Fig. 2.2-6. Experimental results for the peak power of the signal output versus the OPO output reflectivity.
2.3 The influence of birefringence on pulse behavior in OPO
In order to obtain higher output pulse energy, we enlarged the cross section of end surface of lens duct to 3.3 × 3.3 mm2and the diameter of Nd:YAG rod to 6 mm in the setup of Fig.
2.2-1. The initial transmission of saturable absorber, Cr4+:YAG, is designed to be 51%. The QCW pump diode stack consists of six 10-mm-long diode bars generating 130 W per bar, for a total of 780 W at the central wavelength of 808 nm. The coupling efficiency of the lens duct was measured to be approximately 88%. Several output couplers with different reflectivity were used to investigate the output performance and characteristics. Figure 2.3-1 shows the experimental result about the output pulse temporal behavior at 1573 nm for different reflectivity of output coupler, R=9, 16, 34, and 50%, respectively (Wave pro 7100, 10G samples/sec, 1 GHz bandwidth). The output pulse energy ranges from 11 to 9 mJ corresponded to increasing reflectivity. As shown in Fig. 2.3-1, in addition to the first transient peak, another longer pulse was generated adjacent to first one in the cases of reflectivity higher than 16%. With higher output reflectivity, the ratio of second pulse is higher. This is rarely seen in the configuration of external cavity OPO since the generation of fundamental pulse and signal pulse are separated. I will demonstrate in next section that the satellite pulse is resulted from the depolarized fields in the resonator during the process of pulse formation by rate equations. The depolarization is caused by the Nd:YAG rod which acts as a birefringence element under high pump power. The electric fields between the phase-matching direction and the other orthogonal direction couple mutually after each round trip propagation. The energy transformation between two axes exhibits a perturbation in the Q-switch and OPO process. Consequently, the decline and growth of energy in the phase matching direction gives rise to an adjacent parasitical pulse.
Chapter 2 Passively Q-switched Eye-safe Laser with Optical Parametric Oscillator
Fig. 2.3-1. The output temporal pulse at 1573 nm for different reflectivity of output couple R=50%
R=34%
R=9% R=16%
10 ns/div 20 ns/div
20 ns/div R=34% 20 ns/div R=50%
R=9% R=16%
10 ns/div 20 ns/div
20 ns/div 20 ns/div
2.3.1 Theoretical analysis and discussion in birefringence effect
Figure 2.3-2 depicts the typical thermally induced birefringence effect in a Nd:YAG rod with [111] crystal orientation [15]. The principle axes of the induced birefringence are radially and tangentially directed at each point in the rod cross section. In the propagation of electric field along the rod orientation, a linearly polarized field would suffer a phase difference between radial and tangential direction and turn into elliptically polarized. When a linearly polarized field is along y-axis of the rod cross section, it will be gradually depolarized after propagation and resolved into two components along x- and y-axis. This depolarization indicates an energy coupling and transformation between two axes. If the resonator contains a polarizing element to maintain the polarization of fields on y-axis, the field depolarized into x-axis would be filtered out and becomes a depolarization loss. Without any polarizing element in the resonator, on the contrary, the energy will flow back after multiple round trips.
The proportion of energy transformation, Γ, between two orthogonal axes after a round trip could be estimated from the phase difference δ given by [15]
( )r 2π n r( ) 2l dlcr
where λ denotes the optical wavelength, Δn is the difference of refractive index changes between tangential (Δnψ) and radial (Δnr) direction, lcr is the length of Nd:YAG rod, r and φ are the radius and azimuth angle of any interesting point in the cylindrical coordinate system
where λ denotes the optical wavelength, Δn is the difference of refractive index changes between tangential (Δnψ) and radial (Δnr) direction, lcr is the length of Nd:YAG rod, r and φ are the radius and azimuth angle of any interesting point in the cylindrical coordinate system