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Hybridly-pumped continuous-wave optical parametric oscillator I. Breunig, J. Kiessling, B. Knabe, R. Sowade, and K. Buse Institute of Physics, University of Bonn, Wegelerstr. 8, 53115 Bonn, Germany [email protected]

Abstract: We present the first to our knowledge continuous-wave singlyresonant optical parametric oscillator (SROPO) generating tunable signal and idler waves with less than 100 mW single-frequency pump power. This low threshold is achieved by an additional intracavity gain medium that is pumped incoherently. The idler power with respect to the single-frequency pump power shows a bistable behavior which depends strongly on the pumping of the additional amplifier. Furthermore, we demonstrate that such a setup allows a SROPO to be completely diode pumped. © 2008 Optical Society of America OCIS codes: (190.1450) Bistability; (190.4360) Nonlinear optics, devices; (190.4970) Parametric oscillators and amplifiers.

References and links 1. H. Verbraak, A. K. Y. Ngai, S. T. Persijn, F. J. M. Harren, and H. Linnartz, “Mid-Infrared continuous wave cavity ring down spectroscopy of molecular ions using an optical parametric oscillator,” Chem. Phys. Lett. 442, 145–149 (2007). 2. W. R. Bosenberg, A. Drobshoff, J. I. Alexander, L. E. Myers, and R. L. Byer, “Continuous-wave singly resonant optical parametric oscillator based on periodically poled LiNbO3 ,” Opt. Lett. 21, 713–715 (1996). 3. A. V. Okishev and J. D. Zuegel, “Intracavity-pumped Raman laser action in a mid-IR, continuous-wave (cw) MgO:PPLN optical parametric oscillator,” Opt. Express 14, 12169–12173 (2006). 4. I. Paiss, S. Festig, and R. Lavi, “Narrow-linewidth optical parametric oscillator with an intracavity laser gain element,” Opt. Lett. 21, 1652–1654 (1996). 5. I. B. Zotova, Y. J. Ding, X. Mu, and J. B. Khurgin, “Reduction of threshold for a mid-infrared optical parametric oscillator by an intracavity optical amplifier,” Opt. Lett. 28, 552–554 (2003). 6. L. A. Lugiato, “Theory of optical bistability,” in Progress in Optics, Vol. XXI, E. Wolf edt. (Elsevier, Amsterdam, 1984), pp. 69–216.

1. Introduction Continuous-wave (cw) optical parametric oscillators (OPOs) are outstanding light sources because of their large tuning range and narrow linewidth. They are for example used to generate monochromatic light in the mid-infrared for spectroscopic applications [1]. Singly-resonant systems based on periodically-poled lithium niobate (PPLN) and pumped with monochromatic light of wavelengths around 1 μ m have been realized in the past [2]. They require pump powers of at least 500 mW [3]. Although in a multi-resonant OPO the oscillation threshold is significantly lower, it would be favorable to use its singly-resonant counterpart since the stabilization is less complex there. A reduction of the pump threshold in such a system can be achieved by an intracavity laser gain medium. This method has been demonstrated for pulsed singly-resonant optical parametric oscillators (SROPO). Paiss et al. observed no reduction of the oscillation threshold compared with a free-running OPO but a narrowing of the linewidth of the signal #93248 - $15.00 USD

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Received 27 Feb 2008; revised 31 Mar 2008; accepted 2 Apr 2008; published 7 Apr 2008

14 April 2008 / Vol. 16, No. 8 / OPTICS EXPRESS 5662

output [4]. Zotova et al. reported an oscillation threshold of one third compared with their standard picosecond SROPO [5]. In this experiment Nd:YVO was used as the additional amplifier while the idler tuning was performed through changing the pump wavelength as well as the crystal temperature and the quasi-phase-matching (QPM) period of the PPLN. In this article we present a cw SROPO based on periodically-poled lithium niobate applying a phosphate glass doped with Erbium and Ytterbium as the intracavity laser gain element. We show that the oscillation threshold is reduced by more than one order of magnitude. Signal and idler wavelengths are tuned by simply changing the PPLN temperature. Furthermore, a bistable behavior occurs in the idler power as a function of the OPO pump power depending on how strongly the glass is pumped. 2. Experimental setup Our experimental setup, sketched in Fig. 1, comprises a singly-resonant OPO in a bow-tie configuration with two concave mirrors (curvature radius 100 mm) and two plane ones, all with reflectivities of more than 99.9 % in a wavelength range of 1400 to 1800 nm. A MgO-doped PPLN crystal (HC Photonics Corp.), measuring 50 × 8.2 × 0.5 mm 3 , with 7 QPM periods from 28.5 to 31.5 μ m in 0.5 μ m increments, acts as the nonlinear medium. To minimize losses and Fabry-Perot effects, the end surfaces are antireflection coated with a residual reflectivity < 1 % at signal and < 5 % at pump wavelengths. The crystal is positioned on an oven and either pumped with a single-frequency cw Yb:YAG laser at 1030 nm (VersaDisk, ELS) or a singlefrequency cw laser diode at 976 nm (G08 CoS, Bookham). The three interacting waves (pump, signal and idler) are polarized along the optical axis of the nonlinear crystal. An Erbium- and Ytterbium-doped phosphate glass (QX/Er from Kigre Inc.) is placed at the second focal spot of the ring cavity between the plane mirrors. A fibercoupled multimode laser diode at 941 nm with a bandwidth of 5 nm pumps this laser gain medium. Since the oscillator is pumped by two light sources simultaneously, a single-frequency one for the nonlinear crystal and an incoherent one for the glass, we call the system “hybridly-pumped”. We record the spectra of signal and idler fields with a Burleigh WA-650 spectrum analyzer combined with a Burleigh WA-1500 wavemeter. The power of the idler wave is measured after being separated from the residual pump by a dichroic mirror. P* p

PPLN crystal Pp

Pi

lp li

Ps P* Diode

ls

l Diode

Power meter

I l

Er- & Ybdoped glass

Spectrum analyzer

∗ Fig. 1. Schematic illustration of the experimental setup: Pp , Pp∗ , Ps , Pi , and PDiode represent powers of the single-frequency pump, its transmitted portion, the signal wave, the idler wave, and the incoherent pump wave, respectively. Their wavelengths are denoted accordingly.

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3. Results and discussion 3.1. Drastic reduction of the single-frequency pump threshold The main goal of the hybridly-pumped oscillator (HyPO) is to reduce the required singlefrequency pump power compared with that of standard SROPOs. Furthermore, it would be favorable to achieve this over a wide wavelength range in order to maintain the simple tuning scheme at low pump powers. To characterize the performance of the additional amplifier, the nonlinear crystal is pumped by the Yb:YAG laser, and the oscillation threshold is measured for different glass pumping ∗ is powers PDiode . Here PDiode denotes the power actually reaching the amplifier whereas PDiode ∗ the output power of the laser diode: PDiode = 0.84 PDiode . This relation is mainly given by the imperfect transmission of the plane mirror at 941 nm. The single-frequency power threshold value is determined by the occurrence of a peak at the idler wavelength in the output spectrum. We perform this measurement for different crystal temperatures (80 to 200 ◦ C) at a QPM period of 30.0 μ m leading to signal and idler wavelengths of 1525 to 1620 nm and 2845 to 3175 nm, respectively. Idler wavelength [nm] 3000

3150

Threshold [W]

10

2850 Without glass PDiode = 0.00 W PDiode = 0.45 W

1

PDiode = 0.78 W PDiode = 1.10 W

0.1

PDiode = 1.45 W II

0.01

1540

I

PDiode = 2.10 W

1560 1580 1600 Signal wavelength [nm]

Fig. 2. Measured single-frequency oscillation threshold versus signal wavelength for different glass pumping powers (, , , , ◦, • ) and without additional amplifier (×). The solid lines connect the data points as a guide to the eye.

The results are displayed in Fig. 2, and two regions can be distinguished (I and II). At signal wavelengths larger than 1575 nm (region I) the intracavity laser gain medium increases the oscillation threshold by a factor of 1.5 to 3 compared with the one without glass. Even strong incoherent pumping does not reduce the threshold below the original value of about 1 W. In this wavelength range reflections at the glass surfaces are mainly responsible for the additional cavity losses. They cannot be compensated by pumping the laser gain medium since no amplification of the signal wave is provided. However, the picture is completely different for region II: Here the threshold values vary by more than two orders of magnitude. In the case of an unpumped glass, single-frequency powers of up to 10 W are needed to start parametric oscillation. At a given signal wavelength stronger incoherent pumping reduces the threshold significantly. In the wavelength range where the signal field is amplified within the doped glass by stimulated emission (about 1535 to 1565 nm) the single-frequency pump power needed is less than 150 mW at PDiode = 2.1 W. The lowest #93248 - $15.00 USD

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value reached is 20 mW for a signal wavelength of 1545 nm. It should be noted that the determined thresholds define upper limits of the actual ones because the spectrum analyzer needs idler powers larger than 0.5 mW for proper operation. 3.2. Bistability The standard OPO shows a linear increase of the idler power Pi with the single-frequency pump power Pp (Fig. 3(a)). However, after placing the glass amplifier into the cavity we observe a bistable behavior of Pi versus Pp at signal wavelengths between 1535 and 1565 nm, exemplified high in Fig. 3 for λ s = 1545 nm. The oscillation starts at Pp = 7.2 W if the laser gain medium is not pumped at all. At this point Pi grows abruptly to 800 mW and subsequently increases with higher single frequency-pump powers. Yet, with a running oscillation and then decreasing P p also Pi is reduced and the parametric oscillation is maintained until Pplow = 3 W. The occurrence of this bistability can be explained by the laser gain medium acting as a saturable absorber in a resonant cavity [6]. While Pplow is almost independent of PDiode , a larger glass pump power high

reduces Pp significantly (see Fig. 3(b)). At PDiode ≥ 0.5 W no hysteresis is observable anymore. Furthermore, for Pp ≥ 2 W the idler power of the hybridly pumped system does not exceed the values determined for a standard OPO. This indicates that in this region the parametric signal gain is larger than the one provided by the additional amplifier. Another interesting feature occurs which is currently under investigation: For strong pumping of the glass amplifier and weak single-frequency pump Pp we observe a nonlinear dependence of the idler power on P p (see Fig. 3(c)).

Idler power [W]

0.4 0.0 0.8

8

b)

Without glass PDiode = 0.00 W

Threshold [W]

0.8


0.0 0.8 0.4 0.0 0

c)


c) 2

6

4 6 Pump power [W]

8

Without glass Pp high Pp low

4 2 0 0

0.4

Idler power [mW]

a)

80

0.5 1 1.5 2 Diode power [W] Without glass PDiode = 2.10 W

40 0 0

0.5 1 1.5 2 Pump power [W]

Fig. 3. a,c) Idler power versus single-frequency pump power at a signal wavelength of 1545 nm (PPLN temperature of 110 ◦ C) for different values of PDiode (, , •) and without laser gain medium (×). The solid lines connect the data points as a guide to the eye. b) Measured single-frequency oscillation threshold versus diode pump power at a signal wavelength of 1545 nm. Here and ♦ represent the upper and lower thresholds respectively. The dashed line shows the oscillation threshold of a standard SROPO.

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3.3. All-diode-pumped cw singly-resonant optical parametric oscillator In the paragraphs above it has been demonstrated that single-frequency pump powers below 500 mW are sufficient in the hybridly-pumped system to achieve tunable parametric oscillation. Therefore, an entirely diode-laser-pumped HyPO is realized. The PPLN crystal (QPM period 29 μ m) is pumped by 500 mW of a cw single-frequency laser diode at 976 nm. Wavelength tuning is once more realized simply by varying the crystal temperature. We observe optical parametric oscillation from 1535 to 1568 nm for the signal and 2585 to 2680 nm for the idler wave (Fig. 4). The signal tuning range matches the amplification range of the doped glass. The frequency stability of the idler output is measured to be better than ±150 MHz over 40 min at 2647.89 nm without any mode hops. We achieve idler powers from 3 to 15 mW depending on the wavelength with a stability of better than ±15% over 40 min.

Wavelength [µm]

2.68 2.64

Idlerwave

2.60 1.57 1.55

Signalwave

1.53 60

63

66 69 72 Crystal temperature [°C]

75

Fig. 4. Wavelength tuning of the completely diode-pumped HyPO by varying the PPLN temperature.

4. Conclusions We have demonstrated that a continuous-wave singly-resonant optical parametric oscillator (SROPO) with an incoherently pumped intracavity laser gain medium generates tunable signal and idler waves requiring less than 100 mW single-frequency pump powers. This corresponds to a threshold reduction of more than one order of magnitude in comparison with that of a standard SROPO. Depending on the pump power feeding the glass amplifier a bistable behavior is observed, most likely because of saturable absorption. Furthermore, we have shown experimentally that it is possible to use an unamplified cw single-frequency laser diode as a pump source for the nonlinear crystal. This allows a singly-resonant system to be compact and portable. Acknowledgements We thank Guenter Huber and Christian Kraenkel for helpful information about the laser gain medium. Financial support of the Deutsche Forschungsgemeinschaft (FOR 557) and Deutsche Telekom is gratefully acknowledged.

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