Chapter 5 Intracavity Raman Laser with a Vanadate Crystal 62
5.2 Efficient High-Peak-Power Diode-Pumped Actively Q-Switched
Neodymium-doped yttrium aluminum garnet and yttrium othovanadate (Nd:YAG and Nd:YVO4) are the most widely used materials of solid-state laser gain medium [10, 15]. The Nd:YVO4 crystal has a strong broadband absorption and an effective stimulated emission cross section which is five times larger than Nd:YAG [16]. These properties imply that a Nd:YVO4 laser usually has the higher efficiency and broader operating temperature than a Nd:YAG laser. However, the Nd:YAG crystal has other advantages such as better thermal property and much longer fluorescence lifetime. In particular, the long lifetime can raise the output pulse energy of a fundamental actively Q-switched laser and should be able to raise the output pulse energy of an actively Q-switched Raman laser at the Stokes wavelength. This means that the conversion efficiency of the intracavity SRS might be increased in a actively Q-switched Nd:YAG solid-state laser.
In the section of 5.1, we have exhibited the actively Q-switched Nd:YVO4
Raman laser with a YVO4 crystal. In this work, we report the high-pulse-energy and high-peak-power intracavity YVO4 SRS generation in a compact diode-pumped actively Q-switched Nd:YAG laser.
5.2.1 Experimental Setup
Figure 5-5 depicts the experimental configuration for the diode-pumped actively Q-switched Nd:YAG/YVO4 Raman laser. The cavity mirrors which have special dichroic coating for efficient conversion at the first-Stokes wavelength form a plano-concave configuration. The input mirror is a 500-mm radius-of-curvature concave mirror with antireflection coating at 808 nm on the entrance face (R<0.2%), high-reflection coating at 1050~1200 nm (R>99.8%) and high-transmission coating at 808 nm on the other surface (T>90%). The output coupler is a flat mirror with high-reflection coating at 1064 nm (R>99.8%) and partial-reflection coating at 1176 nm (R=51%). Note that the output coupler reflectivity is not optimized and it is limited in availability.
Fig. 5-5. Experimental setup of a diode-pumped actively Q-switched Nd:YAG/YVO4 Raman laser at 1176 nm.
The pump source is an 808-nm fiber-coupled laser diode with a core diameter of 800 μm, a numerical aperture of 0.16 and a maximum output power of 25 W. A focusing lens system with an 85% coupling efficiency is used to re-image the pump
beam into the laser crystal. The waist radius of the pump beam is approximately 400 μm. The laser medium is a 0.8-at.% Nd3+:YAG crystal with a length of 10 mm. Both sides of this laser crystal are coated for antireflection (AR) at 1.06 μm (R<0.2%). The Raman crystal is an a-cut YVO4 crystal with a length of 9.6 mm. These two crystals are both wrapped with indium foil and mounted in water-cooled copper blocks individually. The water temperature is maintained at 20 oC. The 30-mm-long acousto-optic Q-switch (NEOS Technologies) has antireflection coatings at 1064 nm on both faces and is driven at a 27.12 MHz center frequency with 15 W of RF power.
The overall laser cavity length is around 9 cm. The thermal detector (Coherent LM-10 HTD) is used to measure the average power of fundamental and Stokes output.
5.2.2 Experimental Results and Discussions
The actively Q-switched Nd:YAG laser performance at 1064 nm is firstly studied for evaluating the conversion efficiency of the intracavity SRS. For this investigation, an output coupler with partial reflection at 1064 nm is used instead of the above-mentioned Raman cavity output coupler. The optimum reflectivity of the output coupler is found to be approximately 80%. The pump threshold for 1064-nm oscillation is below 2.5 W and insensitive to the pulse repetition rate. With an incident pump power of 16.2 W, the average output powers at 1064 nm are 5.8-6.3 W for pulse repetition rates in the range of 20-50 kHz.
In addition, the partially polarized laser beam with a polarization ratio of 3:1 should be excited by the acousto-optic Q-switch and nonlinear Raman crystal.
However, when the intracavity Raman laser is used, the relatively random polarization may benefit the robustness of the Raman crystal compared to Nd:YVO4/YVO4 Raman laser reported in the section 3-2 [17]. Therefore, our experimental results reveal that the same Raman crystals can sustain higher peak power per unit area in Nd:YAG laser.
In other words, the maximum operating pump power becomes higher. The experimental result for optical spectrum of the Raman laser is monitored by an optical spectrum analyzer (Advantest Q8381A, including a diffraction grating monochromator) with a resolution of 0.1 nm. The first Stokes wavelength of 1176 nm is converted from the fundamental wavelength at 1064 nm by Raman shift at 890 cm-1 from an YVO4 crystal [9]. There is no second Stokes wavelength observed for all pumping power.
Figure 5.6 illustrates the average output power at 1176 nm with respect to the incident pump power for pulse repetition rates of 20, 30, and 50 kHz. For the Raman operation with a fixed pulse repetition rate and a fixed cavity length, a critical pump power or a maximum operating pump power exists. The thermal lensing effect will cause the cavity to be unstable beyond the critical pump power. Reducing the pulse repetition rate leads to a lower threshold for stimulated Raman output, but it leads to a smaller critical pump power and a smaller maximum output power because of the increased thermal loading of the end-pumped Q-switched Nd-doped laser [18].
However, the meaning behind the critical pump power may not be all the same as normal actively Q-switched Nd-doped lasers. Compared with the fundamental operation at 1064 nm, the much lower optical-to-optical conversion efficiency for Raman operation may cause much higher thermal loading on the gain medium. The
additional thermal lensing effect from SRS on a Raman crystal should also be considered. Then, the SRS conversion efficiency could be sensitive to the cavity stability, which will induce the drop in critical pump power. Moreover, the self-focusing-induced damage on the YVO4 crystal never occurs at low pulse repetition rate while the pump power is overdriven.
Input power at 808 nm (W)
0 2 4 6 8 10 12 14 16 18
Average output power at 1176 nm (W)
0.0
Input power at 808 nm (W)
0 2 4 6 8 10 12 14 16 18
Average output power at 1176 nm (W)
0.0
Fig. 5-6. Average output power at the Stokes wavelength of 1176 nm with respect to the incident pump power at various pulse repetition rates of 20, 30, and 50 kHz.
Nevertheless, the efficient average output powers at 1176 nm are 1.6, 2.5, and 3.0 W at pulse repetition rates of 20, 30, and 50 kHz with the incident pump power of 7.6, 13.3, and 16.2 W, respectively. As a consequence, the maximum SRS conversion efficiency of 62-47% with respect to the output power that is available from the fundamental laser of 1064 nm is demonstrated at the pulse repetition rate of 20~50 kHz. Then, the maximum optical-to-optical conversion efficiency from 808-nm pump are as high as 21.3~18.3% at the pulse repetition rate of 20~50 kHz. The overall
conversion efficiency is the second-highest one for Raman lasers until now to our knowledge [19, 20]. The efficiency could be improved if we can use a c-cut YVO4
crystal [12] with properly antireflection coating and an optimized output coupler.
From the average output power and the pulse repetition rate, the pulse energy Ep at 1176 nm with respect to the incident pump power is depicted in Fig. 5-7. The maximum pulse energies Emax at pulse repetition rates of 20 and 30 kHz are both higher than 83 μJ. The Emax of this Nd:YAG/YVO4 Raman laser is almost two times the Emax of the Nd:YVO4/YVO4 Raman laser in almost the same actively Q-switched scheme reported in the section of 3-2 [17].
Input power at 808 nm (W)
0 2 4 6 8 10 12 14 16 18
Pulse energy at 1176 nm (μJ)
0
Input power at 808 nm (W)
0 2 4 6 8 10 12 14 16 18
Pulse energy at 1176 nm (μJ)
0
Fig. 5-7. Pulse energy at 1176 nm with respect to the incident pump power at various pulse repetition rates of 20, 30, and 50 kHz.
The temporal behaviors were recorded by a LeCroy digital oscilloscope (Wavepro 7100, 10 GS/s, 1-GHz bandwidth) with two fast InGaAs p-i-n photodiodes. An interference filter allowing transmission only at 1064 nm is used for extracting weak
fundamental signal. Figures 5-8 shows the pulse shape for the fundamental and Raman components. With a pulse energy of 81.2 μJ at 1176 nm, the pulse width of the pulse envelope in Fig. 5-8 is 2.2 ns, and the peak power of the pulse seen as a Gaussian shape without oscillation should be 36.9 kW. However, the observed mode-locked phenomenon causes the Raman pulse to assume a deeply modulated shape, and peak power Ppeak cannot be approximated as the Ep divided by the envelope width same as Nd:YVO4/YVO4 Raman laser [17]. We have to employ the temporal shape of a single pulse in arbitrary unit, ( )ψ t , to determine the correct Ppeak. Note that the screen shots and numerical data of ( )ψ t are simultaneously recorded in the oscilloscope so that we could use the numerical data for calculation, which can be expressed as Ppeak = ⋅C ψmax( )t , where the ψmax( )t indicates the peak value of ( )ψ t . Factor C is determined by the fact that ( )
p T
E = ⋅C
∫
ψ t dt, where T indicates the temporal range of the single pulse. Consequently, the peak power of the Raman pulse in Fig. 5-8 is 43.5 kW.5 ns/div
Fundamental (1064 nm)
Raman (1176 nm)
2.2 ns
5 ns/div
Fundamental (1064 nm)
Raman (1176 nm)
5 ns/div
Fundamental (1064 nm)
Raman (1176 nm)
2.2 ns 2.2 ns
Fig. 5-8. Oscilloscope trace with mode-locking effect for fundamental and Raman pulses. The figure is recorded with an incident pump power of 7.6 W at a pulse repetition rate of 20 kHz.
5 ns/div (a)
(b)
(c)
5 ns/div (a)
(b)
(c)
Fig. 5-9. Evolvement of pulse shapes dependent on pump power at a pulse repetition rate of 50 kHz.
With incident pump power of 8.6 W, 12.3 W, and 14.8 W, the peak power is (a) 560 W, (b) 13.1 kW, and (c) 15.5 kW.
It can also be observed that the pulse shape and modulated oscillation depth are varied with increased pump power. For example, Fig. 5-9 depicts the typical evolvement of pulse shapes that are dependent on pump power at 50 kHz. Note that the vertical axes of the pulses are separate arbitrary units. With a pump power of 8.6 W, which is near the threshold of SRS, the envelope width of 7.1 ns and deep mode-locked oscillation are showed in Fig. 5-9 (a). Although the pump power is increased to 12.3 W, the envelope width is narrowed to a minimum of 3.1 ns with a Ppeak of 13.1 kW as shown in Fig. 5-9 (b). Meanwhile, the mode-locked oscillation depth decreased when the pump power is increased from the threshold to the 12.3 W.
Then the pulse envelope broadened slightly after the pump power of 12.3W, but the mode-locking effect is enhanced. With a pump power of 14.8 W, the pulse is observed with an almost saturated Ppeak of 15.5 kW as showed in Fig. 5-9 (c). Finally, the pulse-to-pulse amplitude fluctuation of the Raman output is monitored to be approximately less than ±10%.
The temporal behavior and real Ppeak appear to be available. In fact, with further observation, the FWHM of each mode-locked pulse inside the envelope is 500~600 ps.
Narrow mode-locked pulses might be restricted by the 1-GHz bandwidth of the oscilloscope and photo detectors. The restriction is easy to be overlook during the experiment. In other words, the Ppeak could be much higher than what we calculated in our research and in Ref. [17]. However, a conventional autocorrelator for short-pulse measurement of a stable cw mode-locked laser can not provide reliable measurement results for Q-switched mode-locked laser. We estimate that the real-time oscilloscope and photodetectors with an 8~10-GHz bandwidth are required for sure tracing on the time domain. Nevertheless, the high-speed optoelectronic equipment is still being fine tuned and is not yet available for general use. It will be interesting to study the mechanism and behavior of the compact mode-locked actively Q-switched intracavity Raman lasers by use of YVO4 crystals.
Compared with the results from the Nd:YVO4/YVO4 Raman laser in almost the same actively Q-switched scheme, the Nd:YAG/YVO4 Raman laser raises the maximum output pulse energy from 43 to 83 μJ and the peak power from 14 kW to 43.5 kW [17].
5.3 Conclusion
In conclusion, with a YVO4 crystal as a stimulated Raman crystal in a 1064-nm actively Q-switched Nd:YVO4 laser, the maximum operating pump power, repetition rate, average output power, pulse width, and the peak power at the Stokes wavelength of 1176 nm are substantially improved as listed in table 3-1. With an incident pump power of 18.7 W, the average power is 2.61 W at 80 kHz corresponding to the conversion efficiency of 13.9%. The output pulses noticeably display mode-locking phenomena that lead to the maximum peak power to be higher than 10.5 kW at 1176 nm operated at the pulse repetition rate from 20 to 80 kHz. With an incident pump power of 12.7 W, the pulse energy and peak power is higher than 43.5 μJ and 14 kW at 40 kHz.
In addition, it is experimentally demonstrated that the maximum operating pump power and conversion efficiency can be further improved simultaneously by use of a similar Raman laser cavity with another gain medium, Nd:YAG, as listed in table 3-1.
3-W average output power of the efficient diode-pumped actively Q-switched Nd:YAG/YVO4 intracavity Raman laser at 1176 nm is generated at a pulse repetition rate of 50 kHz. The maximum conversion efficiency is up to 21.3-18.3% at a pulse repetition rate of 20-50 kHz. The maximum pulse energy is higher than 83 μJ at both 20 kHz and 30 kHz. The output pulses display mode-locking phenomena that result in a maximum peak power of 43.5 kW.
9
Table. 5-1. Comparison of laser performance between Nd:YVO4 self-Raman laser, Nd:YVO4/YVO4, and Nd:YAG/YVO4 Raman lasers at 1176 nm.
Intracavity Cascade Raman Laser with a KTP or KTA
Crystal
6.1. Introduction
Raman lasers which are based on intracavity stimulated Raman scattering (SRS) in Raman active crystals have a very promising potential for various applications such as pollution detection, remote sensing, and medical system [1-3]. Recently, potassium titanyl phosphate (KTP) and rubidium titanyl phosphate (RTP) which are widely recognized as prominent nonlinear optical crystals involving nonlinear optical susceptibility χ(2) have been experimentally confirmed to be practical SRS converter devices [4-6]. The low value of the KTP-related Stokes shift (270 cm-1) [7] permits generation of multi-frequency radiation with cascade SRS. Nowadays, simultaneous multi-frequency lasing lines with high peak powers in the room temperature is of practical importance for the terahertz (THz) generation with the nonlinear optical difference frequency method [8-10].
Neodymium-doped single vanadate crystals including Nd:YVO4 and Nd:GdVO4
have been identified as excellent laser materials for diode-pumped solid-state lasers because of their large absorption and large emission cross sections [11-18]. In the Q-switching operation, however, the large emission cross sections usually limit their energy storage capacities. To overcome this hindrance, mixed Nd:YxGd1-xVO4
crystals were recently developed with Y ions replacing some of the Gd ions in Nd:GdVO4 single crystal [19-22]. It has been experimentally confirmed that such mixed crystals are substantially superior to single crystals for Q-switching and mode-locking performance because of their broader fluorescence linewidth [22-24].
6.2 Diode-Pumped Multi-Frequency Q-Switched Laser with Intracavity Cascade Raman Emission in KTP Crystal
In this work, we present the first demonstration of a diode-pumped actively Q-switched mixed Nd:Y0.3Gd0.7VO4 laser with an intracavity KTP crystal to produce cascade SRS emission up to the fourth order. With an incident pump power of 14 W, the actively Q-switched intracavity Raman laser, operating at 50 kHz, produces an average output power up to 0.92 W with a pulse energy of 18.4 J. The maximum peak power is generally higher than 2 kW.
6.2.1 Experimental Setup
The schematic diagram for the experimental setup of a diode-pumped actively Q-switched Nd:YxGd1-xVO4 laser with a KTP crystal as an intracavity SRS medium is illustrated in Fig. 6-1. Spontaneous Raman spectral data on KTP crystal reveal that the strongest Raman scattering was observed near 270 cm-1 [7]. With a fundamental pump wavelength of 1064 nm, the first four Stokes lines for the most intense Raman peak can be calculated to be 1096, 1129, 1166, and 1204 nm, respectively. The flat front mirror has antireflection coating (R<0.2%) at the diode wavelength on the entrance face, high-reflection coating (R>99.5%) at the lasing and SRS wavelengths, and high-transmission coating (T>90%) at the diode wavelength on the other surface. The
flat output coupler has the reflectivities R=99.6% at 1064 nm, R=99.1% at 1096 nm, R=86.3% at 1129 nm, R=48.0% at 1166 nm, and R=27.5% at 1204 nm. Note that the present output coupler is selected, but not optimized, from the available mirrors in our laboratory. Nevertheless, experimental results reveal that cascade SRS operation including the first four Stokes components can be obtained.
Laser diode
Coupling lens
R>99.5% at 1060-1200 nm T=85% at 808 nm
R>99.5% at 1060-1200 nm T=85% at 808 nm
Fig. 6-1. Experimental setup of a diode-pumped actively Q-switched Nd:Y0.3Gd0.7VO4 laser with a KTP crystal as an intracavity SRS medium.
The pump source is an 808-nm fiber-coupled laser diode (Coherent Inc.) with a core diameter of 0.8 mm, a numerical aperture of 0.16, and a maximum output power of 16 W. A focusing lens with a 12.5-mm focal length and 85% coupling efficiency is used to reimage the pump beam into the laser crystal. The average radius of the pump beam is near 0.35 mm. The active laser medium is a 0.2-at.% Nd:Y0.3Gd0.7VO4 crystal with a length of 10 mm. The Raman medium is a 20-mm-long KTP crystal with a cutting angle along the x axis (θ=90o and φ=0o). Both sides of the Nd:Y0.3Gd0.7VO4
and KTP crystals are coated for antireflection at 1000–1200 nm (R<0.2%). In addition, they are wrapped with indium foil and mounted in a water-cooled copper block. The water temperature is maintained at 20 oC. The 30-mm-long acousto-optic Q-switch device (NEOS Model 33027-15-2-1) had antireflection coating at 1064 nm on both faces and is driven at a 27.12-MHz center frequency with 15.0 W of rf power.
The present cavity is a flat–flat resonator that is stabilized by the thermally induced lens in the laser crystal. This concept was found nearly simultaneously by Zayhowski and Mooradian [25] and by Dixon et al [26]. A linear flat–flat cavity is an attractive design because it reduces complexity and makes the system compact and rugged. However, the end-pump-induced thermal lens is not a perfect lens, but is rather a lens with aberration. It has been found that the thermally induced diffraction loss is a rapidly increasing function of the mode-to-pump ratio at a given pump power.
When the incident pump power is greater than 5 W, the optimum mode-to-pump ratio is found to be in the range of 0.8-1.0 [27]. Since the laser rod is very close to the front mirror, the laser mode size in the gain medium can be given by [28]
(6-1)
where fth is the effective focal length of the thermal lens,
(6-2)
is the effective cavity length, Lcav is the cavity length, l is the length of the gain medium, n is the refractive index of the gain medium, lKTP is the length of the KTP
( )
( )
1/ 4
1 1/ 4 th l
th
L f L f ω λ
= π
−
KTP KTP Q Q
(1 1) (1 1) (1 1)
L L= cav+l n− +l n − +l n −
crystal, nKTP is the KTP refractive index for the output laser beam, lQ is the length of the Q-switched crystal, and nQ is the refractive index of the Q-switched crystal for the output laser beam. The effective focal length for an end-pumped laser rod can be approximately expressed as
(6-3)
where ωp is the pump size in the unit of mm, Pin is the incident pump power in the unit of watt (W), and C is a proportional constant in the unit of W/mm. The effective focal length at a given pump power can be experimentally estimated from the longest cavity length with which a flat-flat cavity can sustain stable. Therefore, we perform the laser experiments to obtain the critical cavity lengths for different pump powers at a fixed pump size. We use Eq. (6-3) to fit the experimental results and find the constant C to be approximately 1.7×104 W/mm.
Incident pump power (W)
0 5 10 15 20
mode-to-pump size ratio
0.5 0.6 0.7 0.8 0.9 1.0 1.1
Lcav = 160 mm
Lcav = 130 mm
Lcav = 130 mm