Single-frequency pump source operating at 583 THz.

In this section, the operating characteristics of the source of pump frequency for the OPO are considered in detail. This involves a description of the argon-ion laser, and the properties that make it suitable for use as a source of input radiation for the OPO. The steps taken to provide narrow-linewidth output radiation from the argon-ion laser are outlined, and the resulting frequency jitter / linewidth of the pump source is measured and discussed.

The pump source for the OPO described in this chapter was selected to fulfil several criteria, as discussed in more general terms in chapter III. The specific characteristics of the pum p source for this cw OPO included continuous-wave output, single longitudinal mode operation, high power

output, and operation in the visible spectral region, at a frequency near to Vp « 600 T H z, and corresponding to a free-space pump wavelength near to Xp - 0.5 {xm.

Single longitudinal mode operation was required to allow for the possibility of efficient and stable operation of the OPO outputs. High power radiation with single-frequency output at the Watt-level was required to operate a doubly-resonant LBO OPO reliably and consistently aboVe threshold. Frequency operation near to Vp ~ 600 THz was required to operate the OPO with signal and idler frequency outputs in the near infra-red spectral region, and close to a frequency-degeneracy point in the near infra-red with Vg ^ Vj ~ Vp f 2~ 300 THz (i.e. 2Xp « 1 jxm). In recent years, the near infra-red spectral region (X - 1 to 2 |xm) has been the subject of considerable research. Consequently, the quality of optical coatings and nonlinear materials in this spectral region is well documented. This reduces substantially the level of uncertainty with regard to forming a low loss resonator for the proposed frequency-degenerate OPO.

As discussed in chapter II, when the signal and idler frequencies of an OPO are equal, as occurs at the point of exact frequency-degeneracy, the OPO frequency down-conversion process, or energy transfer, is the exact opposite of second harmonic generation. To summarize, a nonlinear material that can be phase-matched to frequency-double a source operating at a frequency of Vp, to provide its second harmonic at a frequency of 2 Vp, is a prime candidate for a frequency-degenerate OPO. In the equivalent OPO configuration, the pump source operates at a frequency of 2Vp and the OPO operates at, and around, frequency degeneracy at Vp, given the same nonlinear material and phase- matching arrangement. Owing to the substantial work reported for the second harmonic generation of near infra-red solid-state lasers operating at frequencies of Vp « 300 THz [4-6], it was decided to operate the OPO under phase-matching conditions that were similar to these frequency-doubling processes. In particular, there has been substantial progress recently when frequency-doubling the radiation from diode-laser-pumped Nd-based lasers (e.g. Nd:YAG and Nd:YLF). This has been due to the excellent spatial and spectral properties that can be provided from these all-solid-state laser sources operating in the near infra-red spectral region [7 - 9], and the desire to produce efficiently harmonics of these frequencies in different spectral regions (e.g. visible and ultra-violet) where such high quality optical radiation is difficult to produce, or does not exist [4,10].

Therefore, the requirement was for a pump source that operated at a frequency near to Vp ~ 600 THz. Such sources emit radiation in the blue / green spectral region, and operate w ith w avelengths near to Xp - 0,5 jxm. Taking into consideration the requirements of the pump source w ith respect to the efficient and reliable operation of a frequency and amplitude stable cw OPO, there are a limited number of laser pump sources that can be considered as the pump source for such an OPO. These include gas lasers that can be forced to operate on single transition lines, or near infra­ red solid-state lasers, when frequency up-converted to the green spectral region. Suitable gas lasers include argon-ion [11,12] and krypton-ion lasers [13,14]. Argon-ion lasers have a number of transition lines in the blue / green spectral region, w ith the highest gains at frequencies (wavelengths) of 615 THz (488 nm), 583 THz (514.5 nm) and 568 THz (528 nm). Krypton-ion lasers have a strong lasing line at 565 THz (531 nm). Suitable solid-state pum p sources include diode-laser-pum ped NdiYAG [8] and Nd:YLF [15] lasers, frequency-doubled with crystals of LBO [6], KTP [5], or MgOiLiNbOg [4], and operating at frequencies (wavelengths) of 564 THz (532 nm) and 574 THz (523 nm), respectively.

The pump source that was selected for the OPO was an argon-ion laser operating at a frequency of Vp » 583 THz. It was chosen as a convenient source of high power, narrow-linewidth, single-frequency radiation when operating at a pump frequency of Vp ~ 600 THz. (Specifically, the argon-ion laser was the only source available to work with in the laboratory when this experiment was performed.) Therefore, the argon-ion laser was suitable as a pump source for this OPO arrangement when operating in the green spectral region. Moreover, this particular laser source could also be operated efficiently and reliably w ith high cw output power levels at higher frequencies, in the ultra-violet spectral region, where Vp « 900 THz. One of these higher frequencies would be used as the pump source for further cw OPO work; see chapters V and VI. The argon-ion laser used was a modified, commercial laser product that was manufactured by Spectra-Physics (model 2045-E).

The Spectra-Physics model 2045-E argon-ion laser (hereafter referred to as the pum p source) was configured as a linear, standing-wave resonator. The cavity m irrors were highly-reflecting over a broadband range (AA« 70 nm [16]) that encompassed several of the blue / green transition lines. Through the use of an intra-cavity dispersing prism, the argon-ion

laser operated on just one transition line, corresponding to a wavelength of Xp = 514.5 nm. (Dispersion within the prism allows only one line to be perfectly aligned with the high-reflector.) Figure IV. 1 displays a schematic representation of the laser source.

Output coupler Aperture

Etalon

Plasma tube bore Prism

Resonator axis Brewster window

High reflector Figure IV. 1.

Schematic representation of the argon-ion laser resonator, used as the source of input pump frequency for the OPO. The intra-cavity prism ensures single-line emission, and the intra-cavity étalon provides single longitudinal mode output.

In general, the gain-b and widths of gas lasers exhibit significant broadening due to the effects of homogeneous and inhomogeneous broadening [17]. For an argon-ion laser, operating on a single transition line at a frequency of Vp = 583 THz, or a w avelength of Xp = 514.5 nm, the frequency gain- bandw idth is Av^_^ « 6 GHz [17], due prim arily to the effects of inhomogeneous broadening: Doppler broadening of spectral lines due to high plasma temperatures, magnetic fields via the Zeeman effect, and pressure and radiation broadening.

Therefore, for such a linear, standing-wave optical resonator, with a cavity length of L~ 1.78 m, corresponding to an axial mode spacing, or free spectral range of FSR « 84 MHz, then without any intra-cavity frequency selective elements, the output consisted of a number of longitudinal modes. (Only if the optical length, L, of the cavity is made sufficiently short, where c f 2L> AVg_^, or L < 25 mm, will this laser oscillate on a single frequency.)

However, in general, longer lengths of laser gain media are employed in commercial argon-ion lasers to obtain greater output powers.

Therefore, the spectral output was found to be a rapidly fluctuating function of time, consisting of a number of longitudinal modes with random amplitude and phase. To be suitable for use as a pump source for the cw OPO, it was necessary to reduce the linewidth of the output, by suppressing the unwanted longitudinal resonances to obtain single frequency output. This was achieved simply by placing a tilted étalon within the resonator, to select one of the axial modes under the gain profile. The air-spaced étalon used was

11 mm in thickness and consisted of two thin, fused-silica, dielectric-coated windows separated by a hollow, low-expansion tube made also of fused- silica [16]: fSKgf^/on~14GHz > Av^_^. Single-frequency operation was obtained by tilting the étalon slightly off the axis of the resonator. The use of an intra­ cavity étalon has the advantages of simplicity of fabrication, relative insensitivity to vibration, and a low insertion loss resulting in a high efficiency. If the étalon is positioned so that the loss generated by the étalon is a minimum in the centre of the gain curve, the modes that can oscillate are reduced to those where the gain exceeds the combination of étalon loss and output coupling loss, or about » ±2 GHz, as displayed in figure IV. 2.

In addition to the étalon, there is another mechanism that further reduces the number of modes which oscillate. The unsaturated gain curve applies only when the laser first begins to lase. As the laser oscillation builds, it reaches intensities which significantly reduce the population difference between the upper and lower laser states, with the result that the gain at the frequency of oscillation is reduced, or saturates, to a level that matches the losses that occur: hole-burning.

Longitudinal modes are close enough in frequency that, in a fully operating laser, adjacent modes will compete with one another. This mode competition arises from the fact that a given excited ion, which nominally will be stimulated at a specific frequency (related to its energy, relative velocity and any Zeeman effects) can undergo stimulated emmision over a narrow range of frequencies that covers several longitudinal modes. Any mode w ithin this range which is slightly favoured over the others will essentially use all the gain available and the other neighbouring modes will disappear. Therefore, the étalon finesse is chosen to provide a broad-band

intra-cavity filter to allow for power to be transferred from adjacent modes near line-centre into the most-favoured mode.

Ion laser unsaturated

gain curve. Single-frequency _output.

a.

Frequency offset

(front line centre). _{Modes suppressed} by étalon. Loss added by étalon. Output coupling loss-line Figure IV. 2

Single-frequency selection using an étalon for the 514.5 nm line.

Single-frequency operation was m onitored on a scanning confocal interferometer = 2 GHz). Free-running, single-frequency operation of the laser is shown in figure IV. 3. Typically, single-frequency power levels in excess of ~ 5 W could be achieved in this manner, representing a conversion efficiency of ~ 50 % from multi-mode to single-mode operation.

As explained in chapter 11, it is im portant to evaluate the exact linewidth, or frequency stability, AVp_gfa^, of the pump laser since this plays a significant role in the amplitude and frequency stability of the signal and idler outputs. Therefore, it is worthwhile considering the principal causes of laser frequency instability in such large-frame argon-ion lasers. The laser frequency stability depends alm ost entirely on the stability of the cavity resonance [11,18]. (Frequency variations due to fluctuations in refractive index of the inverted population are not usually observed unless the discharge is electrically noisy [11,18]. At line centre, fluctuations in inverted population have a negligible effect on the cavity resonance [11,18].)

1

Interferometer FSR = 2 GHz.

4

Frequency bandwidth » 2.5 GHz. Figure IV. 3.

Single-frequency operation at a frequency of Vp = 583 THz.

Changes in the ambient temperature and pressure are responsible for most of the long term drift in cavity resonance due to the cavity length and the intra­ cavity étalon thermal expansions. Much more serious are mechanical vibrations/resonances of the cavity and / or the optical bench generated by airborne sound waves. The short-term frequency jitter was determined primarily by cavity vibrations induced by water flow in the laser head. The frequency range was in the low- to mid-audio region (« 20 to 500 Hz) with m echanically-enhanced resonances dom inating [16]. The short-term ( At « 1 second) frequency-jitter was measured to be i^Vp^stab ~ ± 10 MHz (half­ w idth at half-maximum), by observing the fluctuations in the intensity transmitted through the invar-based, stable monitoring external étalon.

Table IV. 1

Characteristics of the input pump frequency. Pump source Argon-ion laser

Frequency 583 THz

Wavelength 514.5 nm

Jitter (~ 1 second) _^^p-stah «±10 MHz

In chapter II, it was shown that the important requirement for the pum p source of a cw OPO was the short term frequency jitter. To recap, line widths at the « MHz-level are usually marginal for stable OPO operation. The consequence of operating the argon-ion laser with a short-term frequency jitter at the 10 MHz-level is, as expected critical, and the effects of this are discussed further in section IV. 5. The important characteristics of the argon- ion laser, used as the source of input pump frequency for the cw OPO, are summarized in table IV. 1.