Continuous-wave, two-crystal, singly-resonant ... - OSA Publishing

Continuous-wave, two-crystal, singly-resonant optical parametric oscillator: Theory and experiment G. K. Samanta,1,* A. Aadhi,1 and M. Ebrahim-Zadeh2,3 1 Theoretical Physics Division, Physical Research Laboratory, Navarangpura, Ahmedabad 380 009, India ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain 3 Institucio Catalana de Recerca i Estudis Avancats (ICREA), Passeig Lluis Companys 23, Barcelona 08010, Spain * [email protected] 2

Abstract: We present theoretical and experimental study of a continuouswave, two-crystal, singly-resonant optical parametric oscillator (T-SRO) comprising two identical 30-mm-long crystals of MgO:sPPLT in a fourmirror ring cavity and pumped with two separate pump beams in the green. The idler beam after each crystal is completely out-coupled, while the signal radiation is resonant inside the cavity. Solving the coupled amplitude equations under undepleted pump approximation, we calculate the maximum threshold reduction, parametric gain acceptance bandwidth and closest possible attainable wavelength separation in arbitrary dualwavelength generation and compare with the experimental results. Although the T-SRO has two identical crystals, the acceptance bandwidth of the device is equal to that of a single-crystal SRO. Due to the division of pump power in two crystals, the T-SRO can handle higher total pump power while lowering crystal damage risk and thermal effects. We also experimentally verify the high power performance of such scheme, providing a total output power of 6.5 W for 16.2 W of green power at 532 nm. We verified coherent energy coupling between the intra-cavity resonant signal waves resulting Raman spectral lines. Based on the T-SRO scheme, we also report a new technique to measure the temperature acceptance bandwidth of the single-pass parametric amplifier across the OPO tuning range. ©2013 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (190.4360) Nonlinear optics, devices; (190.4400) Nonlinear optics, materials; (190.2620) Harmonic generation and mixing.

References and links 1. 2. 3. 4. 5. 6. 7. 8.

M. Ebrahim-zadeh and M. H. Dunn, “Optical parametric oscillators” in Handbook of Optics (OSA, McGrawHill, 2000), vol. IV, Chap. 22, pp. 1–72. M. Ebrahim-Zadeh, “Continuous-wave optical parametric oscillators” in Handbook of Optics (OSA, McGrawHill, 2010), vol. IV, Chap. 17, pp. 1–33. J. E. Schaar, K. L. Vodopyanov, and M. M. Fejer, “Intra-cavity terahertz-wave generation in a synchronously pumped optical parametric oscillator using quasi-phase-matched GaAs,” Opt. Lett. 32(10), 1284–1286 (2007). I. Breunig, R. Sowade, and K. Buse, “Limitations of the tunability of dual-crystal optical parametric oscillators,” Opt. Lett. 32(11), 1450–1452 (2007). M. Tang, H. Minamide, Y. Wang, T. Notake, S. Ohno, and H. Ito, “Dual-wavelength single-crystal double-pass KTP optical parametric oscillator and its application in terahertz wave generation,” Opt. Lett. 35(10), 1698–1700 (2010). G. K. Samanta and M. Ebrahim-Zadeh, “Dual-wavelength, two-crystal, continuous-wave optical parametric oscillator,” Opt. Lett. 36(16), 3033–3035 (2011). A. Yariv and P. Yeh, Optical Waves in Crystals (Wiley, 1984). A. Bruner, D. Eger, M. B. Oron, P. Blau, M. Katz, and S. Ruschin, “Temperature-dependent Sellmeier equation for the refractive index of stoichiometric lithium tantalate,” Opt. Lett. 28(3), 194–196 (2003).

#185474 - $15.00 USD Received 15 Feb 2013; revised 19 Mar 2013; accepted 20 Mar 2013; published 10 Apr 2013 (C) 2013 OSA 22 April 2013 | Vol. 21, No. 8 | DOI:10.1364/OE.21.009520 | OPTICS EXPRESS 9520

9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

G. K. Samanta, G. R. Fayaz, Z. Sun, and M. Ebrahim-Zadeh, “High-power, continuous-wave, singly resonant optical parametric oscillator based on MgO:sPPLT,” Opt. Lett. 32(4), 400–402 (2007). S. C. Kumar, G. K. Samanta, and M. Ebrahim-Zadeh, “High-power, single-frequency, continuous-wave secondharmonic-generation of ytterbium fiber laser in PPKTP and MgO:sPPLT,” Opt. Express 17(16), 13711–13726 (2009). S. C. Kumar, G. K. Samanta, K. Devi, and M. Ebrahim-Zadeh, “High-efficiency, multicrystal, single-pass, continuous-wave second harmonic generation,” Opt. Express 19(12), 11152–11169 (2011). G. K. Samanta, G. R. Fayaz, and M. Ebrahim-Zadeh, “1.59 W, single-frequency, continuous-wave optical parametric oscillator based on MgO:sPPLT,” Opt. Lett. 32(17), 2623–2625 (2007). U. Bäder, J.-P. Meyn, J. Bartschke, T. Weber, A. Borsutzky, R. Wallenstein, R. G. Batchko, M. M. Fejer, and R. L. Byer, “Nanosecond periodically poled lithium niobate optical parametric generator pumped at 532 nm by a single-frequency passively Q-switched Nd:YAG laser,” Opt. Lett. 24(22), 1608–1610 (1999). G. K. Samanta and M. Ebrahim-Zadeh, “Continuous-wave singly-resonant optical parametric oscillator with resonant wave coupling,” Opt. Express 16(10), 6883–6888 (2008). A. Henderson and R. Stafford, “Spectral broadening and stimulated Raman conversion in a continuous-wave optical parametric oscillator,” Opt. Lett. 32(10), 1281–1283 (2007). A. V. Okishev and J. D. Zuegel, “Intra-cavity-pumped Raman laser action in a mid IR, continuous-wave (cw) MgO:PPLN optical parametric oscillator,” Opt. Express 14(25), 12169–12173 (2006). J. Kiessling, R. Sowade, I. Breunig, K. Buse, and V. Dierolf, “Cascaded optical parametric oscillations generating tunable terahertz waves in periodically poled lithium niobate crystals,” Opt. Express 17(1), 87–91 (2009). T.-H. My, O. Robin, O. Mhibik, C. Drag, and F. Bretenaker, “Stimulated Raman scattering in an optical parametric oscillator based on periodically poled MgO-doped stoichiometric LiTaO3.,” Opt. Express 17(7), 5912–5918 (2009).

1. Introduction The development of coherent optical sources capable of providing multiple wavelengths with wide and arbitrary tuning and high power can be of great interest for a wide range of applications in spectroscopy, microscopy, frequency metrology, short- and long-wavelength nonlinear conversion, and THz generation, amongst many. Optical parametric oscillators (OPOs) have long been recognized as versatile sources of widely tunable radiation in spectral regions inaccessible to lasers [1, 2]. In the conventional configuration, the OPO produces a pair of signal and idler output waves from an input pump wave, where the three wavelengths are subject to energy conservation (ωp = ωs + ωi, where ωp, ωs, and ωi are the pump, signal and idler frequencies, respectively) and phase-matching (kp = ks + ki, where kp, ks and ki are the wave vectors of pump, signal and idler, respectively). Wavelength tuning across broad spectral regions is achieved by varying the phase-match condition to fulfill energy conservation for a new pair of signal and idler waves. On the other hand, due to the coupling of the signal and idler waves through energy conservation and phase-matching, the generation of truly arbitrary signal-idler wavelength pairs with independent tuning control is not possible. This limitation can have consequences, for example, in the generation of closely spaced wavelengths, where the only solution is to tune the OPO to near degeneracy, with the associated difficulties such as broad spectral bandwidth or output power and frequency instabilities [3]. To overcome the constraints of energy conservation and phase-matching, earlier attempts have included a dual-crystal continuous-wave (cw) OPO [4] or double-pass-pumping of a single-crystal pulsed OPO [5], providing two different signal and idler wavelength pairs with arbitrary tuning. In the former scheme [4], both the crystals are placed in series and the undepleted pump from the first crystal is used as pump for the second crystal. The air gap between the crystals in this scheme is an important parameter to avoid relative phase shift among the interacting waves for efficient parametric generation. In the latter scheme [5], the undepleted pump after the first pass is used to pump the same crystal in the second pass. However, in both techniques, high pump depletion in the first crystal or the first pass, generating the first signal-idler pair, deteriorates the pump beam quality available in the subsequent crystal or pass. As a result, the overall OPO performance in terms of threshold and output power in the second crystal or second pass, producing the second signal-idler pair,


is degraded compared to that in the first crystal or first pass. Additionally, in these schemes, thermal effects and crystal damage issues at higher pump powers are major challenges to overcome. We recently reported a novel and generic approach for the generation of two signal-idler wavelength pairs with truly independent and arbitrary tuning, which are unbound by energy conservation and phase-matching, using a simple design based on a compact fourmirror ring-cavity cw OPO [6]. In this scheme, the two signal-idler wavelength pairs can be independently controlled to provide any arbitrary pair of wavelengths within the available OPO tuning range. Both wavelengths are resonant within the same cavity, thus providing high circulating intensities at the two wavelengths, and the beams have the same optical power and exhibit similar output stability. The cw OPO is based on two identical MgO:sPPLT crystals (30-mm-long), each pumped separately at 532 nm by a single pump laser. We have shown frequency separation of two arbitrary signal wavelengths down to 0.55 THz, demonstrating the potential of the system for tunable THz generation. Moreover, while operating both crystals under the same phase-matching condition, coherent coupling between the circulating signal waves generated by the two crystals results in threshold reduction, enabling OPO operation at reduced pump powers. Here we present the theoretical framework for such a scheme, explaining the maximum threshold reduction, gain acceptance bandwidth and closest possible arbitrary wavelength pair separation that can be generated, and compare the analysis with experimental results. We have experimentally verified the performance of such a scheme at higher overall pump power, without inducing any crystal damage, resulting total output power of 6.5 W for 16.2 W of green pump power. We have confirmed intra-cavity cw parametric amplification of the resonant signal from one crystal in the other crystal, confirming the generic nature of the scheme, implying that the technique can be deployed with different combinations of crystals and pump wavelengths. We also report a new technique to measure the cw parametric gain acceptance bandwidth of the OPO. For completeness of the theoretical and experimental analysis of the two-crystal OPO, we have included some of the results reported in [6]. The present report is divided into six sections, comprising introduction, theoretical analysis, numerical simulation, experimental configuration, experimental results and conclusions. 2. Theoretical analysis The generic configuration of the two-crystal cw OPO is shown in Fig. 1.The OPO is arranged in a ring resonator with two crystals X1 and X2 of lengths L1 and L2, respectively, located at the two focii of the cavity. The OPO is a singly resonant oscillator (SRO), where only the signal waves generated by the individual crystals are resonant inside the cavity. In the generic design of Fig. 1, the crystals X1 and X2 are pumped by two separate pump beams of power Pp1 and Pp2, respectively, and exit the cavity in a single-pass. However, we will also consider the use of a single pump beam, Pp1, for both crystals X1 and X2, corresponding to the schemes used in earlier reports [4, 5]. Let us consider z as the propagation direction, with the entrance and exit faces of the crystal X1 designated as z1 and z2, respectively, and z3 and z4 as the entrance and exit of the crystal X2 respectively. Using coupled-wave equations [7], we can investigate the main operating features of the device including parametric gain enhancement, threshold reduction, and gain acceptance bandwidth under two different schemes characterized by different boundary conditions, as described in section 2.1. All results for dual-crystal SRO (D-SRO) and two-crystal SRO (T-SRO) configurations (defined in section 2.1) are compared to that of the conventional single-crystal SRO (S-SRO).


Fig. 1. Schematic diagram of the SRO along with the notations used in the paper. Green, red and brown colors represent pump, signal and idler, respectively.

2.1 Coupled-wave equations To understand the performance of SRO systems, we calculate the parametric gain using Maxwell’s wave equations with nonlinear polarization P(2) = ε0χ(2)A2 as the source term. Throughout the calculations, we consider the pump, signal and idler fields, Ap,s,i, to be plane waves, and for simplicity we also neglect absorption losses and pump depletion. From Maxwell’s equations, one can derive the coupled wave equations describing the evolution of signal and idler field amplitudes within the crystals, as [7] dA s = − j γ s A p Ai*e − j Δkz dz

(1)

dA i* = j γ i A p* A s e j Δkz dz

(2)

where, j = −1 is the complex number. Ap, As and Ai are the pump, signal and idler complex field amplitudes, respectively; Δk = (ω p n p − ωs n s − ωi n i ) / c − 2π / Λ is the phase-mismatch, with ωp,s,i, and np,s,i the frequencies and refractive indices of the pump, signal and idler fields, respectively; c is velocity of light in vacuum; and γ s ,i = ωs ,i d eff / ( n s ,i c ) is the gain factor, with deff = 1/2 χ(2) the effective nonlinear coefficient. In case of quasi-phase-matching (QPM), Λ is the QPM period length, otherwise it is infinity. The real field amplitudes of the pump, signal and idler radiation can be represented as A p ,s , i

2

=

I p ,s , i 2ε 0 n p ,s ,i c

=

Pp ,s ,i

πw

2 p ,s ,i

ε 0 n p ,s ,i c

,

where Ip,s,i, Pp,s,i and wp,s,i are the intensities, powers, and beam waist radii of the pump, signal and idler, respectively. ε0 is the permittivity of free space. Using the treatment of [7], the signal and idler field amplitudes at the exit of the crystals X1 and X2 can be written as As 1,2 ( L1,2 ) e

j

Δk 1 ,2 L,1 ,2 2



j Δk1,2



2 g1,2

=  cos h( g1,2 L1,2 ) +



γ s 1,2



g1,2

sin h( g1,2 L1,2 )  As 1,2 (0) − j

*

sin h( g1,2 L1,2 ) Ap 1,2 (0) Ai 1,2 (0)

(3)


*

Ai 1,2 ( L1,2 ) e

−j

Δk 1 ,2 L1 ,2 2

= j

γ i 1,2 g1,2



j Δk1,2



2 g1,2

sin h( g 1,2 L1,2 ) Ap 1, 2 (0) As 1,2 (0) +  cos h( g 1, 2 L1,2 ) − *



sin h( g 1,2 L1,2 )  Ai 1,2 (0) (4) *



where, Δk1,2 2

g1,2 =

2

γ s 1,2γ i 1,2 Ap 1,2 (0) −

Δk1,2 2

=

4

Γ1,2 − 2

4

, γ s 1,2 ,i 1,2 =

ωs 1,2 ,i 1,2 d eff 1,2 n s 1,2 , i 1,2 c

and Γ1,2 = γ s 1,2γ i 1,2 Ap 1,2 (0) 2

2

(4a)

Here, suffixes 1 and 2 represent the parameters for crystal X1 and crystal X2, respectively. Ap1,2(0), As1,2(0) and Ai1,2(0) are the pump, signal and idler complex field amplitudes, respectively, at the entrance of the two crystals. We analyze two SRO configurations: (a) Dual-crystal SRO (D-SRO), where crystals X1 and X2 are pumped by a single pump beam, Pp1, and the transmitted pump as well as the signal and idler fields generated by X1 are present at the input to X2; and (b) Two-crystal SRO (T-SRO), where X1 and X2 are pumped separately by Pp1 and Pp2, respectively, and only the signal field generated by X1 as well as Pp2 are present at the input to X2, with the idler generated by X1 exiting the cavity in a single pass. In both cases, the signal field at the entrance of crystal X1 is finite, As1(0), and the idler field at the entrance of X1 is zero, Ai1(0) = 0. We neglect pump depletion and all signal losses due to the cavity mirrors. (a) Dual-crystal SRO (D-SRO) Both signal and idler fields generated in crystal X1 are the input fields at the entrance of X2, i.e., As2(0) = As1(L1), Ai2(0) = Ai1(L1). The pump for crystal X1, Pp1, is also used to pump crystal X2, i.e. Ap2 = Ap1 (negligible pump depletion). No external pump is provided to crystal X2. The pump and idler fields after crystal X2 are out-coupled, while signal field is resonant inside the cavity to maintain SRO condition. Under the above boundary conditions and proper substitution of the field amplitudes of the crystal X1 in Eqs. (3) and (4), the signal and idler field amplitudes at the exit of crystal X2 can be represented as  j Δk L + Δk L  ( 2 2 1 1 ) 2 

As 2 ( L2 ) = As 2 ( L1 + L2 ) exp 

γ γ  j Δk 2   j Δk1  * =  c2 + s2   c1 + s1  As1 (0) − j s 2 s2 × j i1 s1 Ap 2 (0) Ap1 (0) As1 (0) 2 g2 2 g1  g2 g1  

 j Δk L + Δk L  ( 2 2 1 1 )  2 

(5)

Ai*2 ( L2 ) = Ai*2 ( L1 + L2 ) exp  − = j

γ i2 g2



j Δk1



2 g1

s2  c1 +



s1  Ap 2 (0) As1 (0) + j *



γ i1 g1



j Δk 2



2 g2

s1  c2 −



(6) *

s2  Ap1 (0) As1 (0)



where c1,2 = cos h( g 1,2 L1,2 ) and s1,2 = sin h( g 1,2 L1,2 ) . (b) Two-crystal SRO (T-SRO) The signal field at the exit of crystal X1 is considered as the input signal field at the entrance of crystal X2, i.e., As2(0) = As1(L1). The idler field is growing from zero, i.e., Ai2(0) = 0. A new pump field, Ap2, is incident on X2. The pump and idler fields are out-coupled after each crystal, while signal field is resonant inside the cavity to maintain SRO condition. Using Eqs. (3) and (4) with proper substitutions and above boundary conditions, the signal and idler field amplitudes at the exit of crystal X2 can be represented as


 j Δk L + Δk L   c + j Δk 2 s   c + j Δk1 s  A (0) (7) ( 2 2 1 1 ) 2 2  1 1  s1 2 g2 2 g1   2  

As 2 ( L2 ) = As 2 ( L1 + L2 ) = exp  −

 j Δk L + Δk L  j γ i 2 s  c + j Δk1 s  A* (0) A (0) (8) ( 2 2 1 1 ) 2  1 1 p2 s1 2 g1  2  g2 

Ai*2 ( L2 ) = Ai*2 ( L1 + L2 ) = exp 

where, c1,2 = cos h( g 1,2 L1,2 ) and s1,2 = sin h( g 1,2 L1,2 ) .

2.2 Parametric gain We define the net power gain of the signal field to be G ≡

A s (L )

2

A s (0)

2

−1

(9)

Inserting the signal field amplitude, as represented by Eq. (3) for a single crystal of length L, into Eq. (9), the single-crystal gain, GS, can be represented as  sin h 2 ( gL )  G S =  (ΓL ) 2  ( gL ) 2  

(10)

Similarly, inserting the Eq. (5) and Eq. (7) in Eq. (9), the simplified expression for the double-crystal gain (GD) can be written as A s 2 (L1 + L 2 )

GD ≡

A s 1 (0)

2

2

2   2 2 sin h ( g (2L ) ) − 1 = (c 2c1 + s 2 s 1 ) − 1 = 4 ( ΓL )  2  ( g (2L ) ) 

(11)

and for two-crystal gain (GT) as G ≡ T

A (L + L ) s2

1

A (0)

2

2

2

− 1 = (c c 2

1

)

2

s1

(

− 1 = cos h (gL ) + 1 2

) ( ΓL )

2

sin h

2

( gL )

( gL )

2

=2

 ( ΓL )

2

sin h

2

( gL )   (12)

( gL )

2

where, both the crystals are identical, have equal lengths, and are operating at the same phasematching condition (L1 = L2 = L, g1 = g2 = g, Γ1 = Γ2 = Γ and Δk1 = Δk2 = Δk = 0). Under small signal gain approximation (g1L110 W), and roll-off in the pump depletion occurs, as reported earlier [12], due to the thermal effect arising from the higher pump powers. Using a green laser of maximum power 18 W, we further increased the pump power to the S-SRO and measured the idler output power. The idler power increases with the pump power in a similar manner to the results shown in Fig. 13. However, for pump powers beyond 10 W, we observe crystal damage. In repeated trials, we have observed similar bulk damage in the crystal, which can be attributed to thermal effects arising from the focusing of the high pump power into a small beam waist radius (w0~30 μm) at the centre of the crystal. Such crystal damage places an upper limit to the input pump power for the S-SRO under the given experimental conditions of ~10 W. We, however, expect that high-power operation of the SSRO can be obtained using loose focusing, but at the expense of increased threshold. On the other hand, since in the T-SRO we divide the input power into two crystals, the pump power in each crystal is below the damage threshold, even for a total input power of 18 W. As evident from Fig. 14, the total output power of the T-SRO (the sum of the powers at the exit of mirror M2 and M4) increases with the increase in pump power at a slope efficiency of 40%, providing a maximum total power of 6.5 W for 16.2 W of pump power. The corresponding maximum signal and idler powers are 2.2 W and 4.3 W, respectively, which is the highest idler power ever reported using MgO:sPPLT crystal pumped in the green.

Total Output Power (W)

7 6 5

o

T1=70.5 C o T2=73 C

Signal+Idler

4 Idler

3 2

Signal

1 0

8

10 12 14 Total Input Power (W )

16

Fig. 14. Variation of total output power, out-coupled signal power and idler power of the TSRO as a function of pump power at crystal temperatures T1 = 70.5°C and T2 = 73°C. Corresponding signal (idler) wavelength is 980 nm (1163.7 nm). Solid curves are guide to the eye.

It should also be noted that although in the T-SRO the thermal dephasing effect is also distributed with the division of the pump power into the two crystals, thermal effects arising from the higher intra-cavity power, as also reported earlier [14], can still be a major challenge to overcome. As also evident from the Fig. 14, the idler power increases with the pump power while there is no significant increase in the out-coupled signal power. The saturation of the intra-cavity signal power can be attributed to the thermal dephasing effect in the nonlinear crystals at higher intra-cavity power and can, in principle, be lowered by reducing the intracavity power using higher output coupling. The increase in operation threshold, due to higher output coupling and loose focusing of the pump beam to avoid crystal damage, can be


compensated by the threshold reduction effect in the T-SRO. There is no evidence of saturation in the output power, indicating that the T-SRO can be operated at even a higher power. Given this potential, using a single output coupler for the signal, one could possibly extract sufficient signal power in a single signal beam for extra-cavity THz wave generation. 5.8 Power across the tuning range We also measured the total output power as a function of the temperature T1 of crystal X1 across the tuning range, with the results shown in Fig. 15. For each value of T1, we have adjusted the temperature T2 of crystal X2, so that both crystals are operating under coherent coupling condition at the same signal and idler wavelengths. At a total pump power of 16.2 W (with P1 = P2 = 8.1 W), the total output power from the T-SRO varies from 5.52 W at T1 = 64°C to 3.53 W at T1 = 127°C, with a maximum of 6.5 W at T1 = 70.5°C. The reduction in the total output for crystal temperatures away from degeneracy can be attributed to the reduced signal power (few milliwatts) due to the high reflectivity (R>99.9%) of the output mirrors, M2 and M4, higher reflection loss at the crystal coating, and also the gain reduction factor, as reported earlier [12]. Although we have measured the output power up to 127°C to verify the high power performance of the T-SRO without any sign of crystal damage even at lower crystal temperatures, one can in principle extend its operation across the entire tuning range of 850-1430 nm [9]. The pump depletion of both crystals varies by ~70-80% across the tuning range. The signal (idler) wavelength varies from 991 nm (1148.6 nm) to 916 nm (1269 nm) for a change in crystal temperature from 64°C to 127°C. 7 916 nm 1269 nm

5 of T-SRO (W)

Total Output Power

6

4 3

991 nm 1 1 4 8 .6 n m

2 1 0

60 70 80 90 100 110 120 130 o T e m p e ra tu re o f C rys ta l X 1 ( C )

Fig. 15. Variation of total output power of the T-SRO across the tuning range. Both the crystals are pumped with equal amount of pump power (P1 = P2 = 8.1 W). Solid curves are guide to the eye.

6. Conclusions

In conclusion we have theoretically and experimentally analyzed the performance of a twocrystal SRO. Solving the coupled wave equations under undepleted pump approximation, we calculated the maximum threshold reduction of 50%, temperature acceptance bandwidth (FWHM) of 0.9°C, 0.8°C and 0.7°C at crystal temperatures 87.5°C, 139°C and 192°C, respectively, and closest possible arbitrary dual-wave generation with a wavelength separation of 1.39 nm at 91°C. We have measured a lower threshold reduction of 39% compared to the theoretically predicted value, which can be attributed to the additional losses due to the crystal coating, and material absorption. Based on the T-SRO scheme, we also report a new technique to measure the temperature acceptance bandwidth of the single-pass parametric amplifier of 3°C, 2.6°C and 2°C at 87.5°C, 139°C and 192°C, respectively, across the OPO tuning range. The discrepancy between the experimental and theoretical temperature acceptance bandwidth can be attributed to the bandwidth of the intra-cavity seed signal


radiation generated by crystal X1 at pump power above threshold and also the pump power to crystal X2. Although the T-SRO has two identical crystals, the parametric gain in this device has an acceptance bandwidth equivalent to a single crystal. Experimentally, the closest signal wavelength (frequency) separation under arbitrary dual-wavelength operation has been observed to be 1.76 nm (0.55 THz), which also verifies the broadening of the temperature acceptance bandwidth. Due to the division of pump in two crystals, the T-SRO can tolerate a higher total pump power, reducing the risk of optical damage and thermal loading in the crystal. We also experimentally verified the high power performance of such scheme, where we obtained a total output power of 6.5 W for 16.2 W of green power at 532 nm. The intracavity parametric amplification of the resonant signal of one crystal in the other crystal shows the possibility of using different combinations of crystals and pump radiation. We verified the coherent energy coupling between the intra-cavity resonant signal fields resulting Raman spectral emission. Acknowledgments

This research was supported by the Ministry of Science and Innovation, Spain, through project OPTEX (TEC2012-37853) and by the European Office of Aerospace Research and Development (EOARD) through grant FA8655-12-1-2128.
