Erbium–ytterbium microlasers: optical properties and lasing characteristics
Introduction
Since its first operation in the pulsed regime in 1965 [1]the erbium glass laser attracted much interest, in particular for telemetry and laser ranging applications, due to its emission at the eye-safe wavelength of 1530 nm. More recently, with the development at the end of 1980s of the new strained-layer InGaAs laser diodes emitting at 980 nm wavelength [2], laser physicists have in particular been actively considering continuous wave laser oscillators based on Er3+ doped glasses and optical fibers. This interest was also stimulated by the successful operation of the erbium-doped fiber amplifier (EDFA) pumped at 980 nm wavelength [3], which rapidly turned out to be a key component in all modern optical transmission systems. In the last few years many authors have successfully demonstrated pulsed as well as continuous wave (cw) operation of erbium active material in different hosts, such as bulk Er–Yb:phosphate glasses 4, 5, 6, 7, 8, 9, the silicate Er–Yb:Y2SiO5 (YSO) 10, 11, 12, the garnet Er–Yb:Y3Al5O12 (YAG) 12, 13, the oxyapatite Er–Yb:SrY4 (SiO4)3O [14], the Er–Yb:Ce:Ca2Al2SiO7 (CAS) crystals [15], and Er–Yb phosphate-based glass fibers 16, 17, 18, 19, 20. In all the above mentioned erbium-doped materials codoping with ytterbium is generally used to decrease the threshold pump rate, owing to an effective ytterbium to erbium transfer mechanism of the excitation energy. Among the above hosts, however, the silicate crystal exhibits poor thermomechanical properties; the lasing emission of erbium in YAG garnet is shifted at 1.64 μm wavelength, a value beyond the optimal eye-safe wavelength and outside the useful wavelength interval for optical communications; unsatisfactory laser performance was obtained with oxyapatite (1.6 mW output power with high threshold) and with CAS crystals (5.5% slope efficiency in the best experimental conditions). To date, the best laser performance has been obtained with erbium and ytterbium doped glasses.
In this paper we analyze the optical properties of the bulk phosphate glass doped with erbium and codoped with ytterbium with the aim of modeling the active material and design miniaturized laser cavities. We also present a comprehensive review of the work done by our group on diode-pumped bulk Er–Yb microlasers, with particular emphasis toward devices of potential interest for optical communication applications. Different operating regimes and peculiar properties of this novel laser will be described and discussed. The paper is organized as follows. In Section 2we describe the main optical and spectroscopic properties of the Er–Yb:phosphate glass and we discuss a simplified model, based on a rate equation analysis, useful for design and optimization of end-pumped microlasers. In Section 3we present the experimental results obtained with different devices in continuous-wave operation, both in multi-longitudinal mode and single frequency regime, and we analyze the tuning properties of the laser. A wavelength division multiplexing (WDM) transmission experiment based on a monolithic multi-longitudinal mode Er–Yb microlaser is also discussed. Section 4is devoted to the study of the Er–Yb microlaser in pulsed operation, and we present the results obtained in mode-locking and frequency modulation regimes (to generate high-bit-rate picosecond pulse trains) and Q-switching regime (to generate high energy nanosecond pulses). In Section 5we analyze the amplitude noise of the oscillator, and we report on the intensity noise suppression and on the frequency stabilization and locking of the Er–Yb microlaser to molecular lines of acetylene, to be used as absolute frequency references for WDM applications.
Section snippets
Optical properties and modeling of active material
Spectroscopic properties of the erbium ion in several glass hosts have been the object of extensive investigations 21, 22. Erbium-doped glasses act as three-level systems at 1.5 μm and codoping with ytterbium is the most effective way to enhance the absorption and pumping efficiencies 6, 23, 24, 25, particularly in the case of very short cavity length devices (e.g. bulk microlasers or particular fiber lasers 16, 17, 18, 19, 20). In fact, this indirect pumping process, which is based on an
Continuous wave laser operation
Quasi-continuous-wave laser operation of bulk erbium in glass was achieved by end-pumping with a Nd:YAG laser at 1064 nm wavelength [45]; cw operation was subsequently demonstrated by pumping with a cw diode-pumped Nd:YAG laser [4]and, contemporary, by pumping at 980 nm both with a Ti:sapphire laser and with an InGaAs diode laser by our group 5, 6. For any practical application, the InGaAs diode-pumped configuration is by far the most interesting one. In the following of this paragraph we
Pulsed laser operation
Bulk erbium–ytterbium lasers have been successfully demonstrated to operate in pulsed regimes generating either pulses in the picosecond scale at high repetition rates by active mode-locking techniques 68, 69, 70, 71and in the nanosecond scale at low repetition rates by active and passive Q-switching techniques 72, 73, 74, 75, 76, 77. Both the pulsed regimes appear to be of great importance for several applications. In particular, nanosecond pulses around 1530 nm emitted from a compact source
Amplitude and frequency stabilization
In this paragraph we discuss amplitude and frequency stabilization of diode-pumped bulk Er–Yb microlasers discussed in Section 3.2due to their importance for several applications to optical communication as well as to metrology, where stability of the laser source is of particular concern.
Conclusions
We presented a comprehensive analysis of the optical and lasing properties of the Er–Yb:glass material, as well as a complete review of the work done by our group on diode-pumped bulk Er–Yb microlasers. Different operating regimes and peculiar properties of this novel laser device have been described, namely multi-mode and single-frequency operation, frequency tuning and stabilization, intensity stabilization, Q-switching, mode-locking and frequency modulation operation. Due to the high
Acknowledgements
This research was partially supported by a grant from the Italian National Research Council under “Progetto Finalizzato Tecnologie Elettroottiche” and “Progetto Strategico Materiali e Dispositivi per Optoelettronica”. Part of the work was also carried out with the financial support of Italtel.
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