BACKGROUND OF THE INVENTION For well over a decade, Dye Master Oscillators (DMOs) have performed in a crucial role as the ultra-high resolution sources of tunable orange-yellow light to be amplified to the several hundred watt level for isotope separation experiments. At the time of their initial development, the DMOs represented an impressive engineering accomplishment, since they operated at high repetition rates (e. g., several kilohertz) with high beam quality and spectral punty. Their output power was sufficient to drive the high gain, dye amplifier chain to saturation with low ASE.

Over the years, various problems and performance limitations have been noted with DMOs as well as other ancillary wavelength locking and spectral formatting equipment (i. e., waveform generators or WFGs). DMOs are built from custom-made mechanical and optical components. Some of the optical components have long delivery times dye cells) while other components are difficult to adequately test prior to use.

For example, intra-cavity etalons have relatively high reflectivity coatings on two opposing sides, thus making flatness and finesse testing difficult. Furthermore, construction and alignment of a DUO for single mode operation is a challenging task.

Even with a new DMO, the electronic lock loops that are used to achieve single mode operation as well as wavelength locking are typically not very robust, thus making unattended operation for extended periods of time unrealistic.

A number of WFG system problems accompany the use of DMOs. First, since they produce short pulses (i. e., tens of nanoseconds), standard Michelson interferometers cannot measure their wavelengths accurately. Second, synchronization of

the DNIO injection pulses with respect to the dye chain pump pulses must be carefully optimized. Third, pump pulse timing jitter is a problem. Fourth, maintenance of a dye circulation system with high chemical purity and ultra-stable ( 5 mK) temperature control is a requirement for reliable, drift free DMO operation. Fifth, photochemical degradation of the dye solution occurs while the dye is being pumped, thus requiring concentrated dye solution to be periodically added to maintain a specified pump absorption coefficient. Sixth, DMO pump light striking the dye cells eventually causes damage, necessitating repair.

There are several sources of light that can be used as an alternative to the DMO-type WFG in an isotope separation system. For example, pulse amplified CW dye lasers utilizing straightforward spectral and spatial filtering provide satisfactory peak power levels substantially devoid of ASE. These systems, however, typically require substantial dye jet maintenance and often exhibit unstable wavelength locking. A second approach using grazing incidence-style pulsed dye lasers offer the advantage of design simplification, although this approach retains many of the disadvantages associated with short pulse dye lasers. Additionally, this approach does not meet the spectral requirements of a typical isotope separation system. A third approach uses solid-state, rare earth ion lasers. Although some of these lasers can be pumped with laser diodes and/or run in a CW mode. they typically have limited tunability. In a fourth approach, solid-state transition metal ion lasers can be used, preferably operating in the 1. 1 to 1.3 micrometer wavelength range. Such lasers are based mainly on the T2--+ A ? transitions of octahedrally-coordinated Cr3+ and tetrahedrally-coordinated Cr4+. Frequency doubling of these lasers can, in principle, provide the desired process wavelengths, although custom crystal growth may be involved. Since the emission transitions generally have small cross sections and short lifetimes, and because crystal losses are substantial, laser thresholds are high. Development of a line-narrowed version of this laser with spatial mode quality suitable for efficient frequency doubling seems plausible, but remains to be demonstrated. In a fifth approach, external cavity, single spatial mode diode lasers in the 1. 1-1.3 micrometer region might be made to operate cryogenically with hundred milliwatt power levels. Frequency doubling of these devices would provide the desired process wavelengths. Construction of diode lasers for this wavelength range, however, is considered inherently difficult. In a sixth approach, short pulse, optical parametric

oscillators (i. e., OPOs) could be used if the line widths could be sufficiently narrowed.

This would probably require a switch to a doubly resonant design to reduce the oscillation threshold although this could possibly lead to mode hopping and difficulties in tuning.

SUMMARY OF THE INVENTION The present invention provides a tunable visible laser system. The laser system can be used to replace the dye lasers in isotope separation systems. Additionally, the laser system can be used as a source of resonance radiation for process monitors, chemical analysis, LIDAR, spectroscopy, fluorometry, projection displays, and communications.

In one aspect of the invention, the output of at least one diode-pumped, cladded, ytterbium doped single mode fiber laser is mixed with the output of a single Nd : YAG laser. For each Yb: silica laser, a pair ofBragg gratings are used to force it to oscillate at the desired wavelength. The Bragg gratings can either be tuned by temperature or pressure.

In another aspect of the invention, sum frequency mixing is used to mix the outputs of the Nd: YAG laser with the outputs of one or more Yb: silica lasers. The mixing is performed with a non-linear crystal, such as a Lino, crystal or periodically poled lithium niobate crystal.

In another aspect of the invention, a feedback loop is used to control the output of the Nd : YAG and Yb : silica lasers. The system uses a CW wavemeter to measure the output of each laser, the monitored frequency being used to control the individual laser outputs. In at least one embodiment, a portion of the output of the non- linear crystal used to mix the outputs of the lasers is passed through a wavelength division multiplexer and a fiber optic switch, thus allowing a single wavemeter to be used in conjunction with multiple lasers.

In another aspect of the invention, electro-optic modulators are used to broaden the spectrum of the output of the non-linear crystal used to mix the outputs of the lasers. In the preferred embodiment, both phase and amplitude modulators are used.

A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the basic master oscillator source according to the invention ; Fig. 2 illustrates the expected tuning range of a frequency doubling and/or SFG system based on tunable Yb: silica and fixed 1319 nanometer output Nd : YAG lasers; Fig. 3 illustrates a simplified WFG system according to the invention; and Fig. 4 is an illustration of an alternate embodiment of the invention in which the output of the Yb: silica laser is modulated prior to mixing.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS Fig. 1 illustrates the basic master oscillator source according to the present invention. The system is designed to provide three different wavelengths using only two types of sources. Sum frequency mixing (hereafter, SFG) and/or frequency doubling is used to achieve the desired wavelengths as well as the necessary line width.

Lasers 101-103 shown in Fig. 1 are Yob3+ : silica fiber lasers. Preferably a master oscillator-power amplifier (i. e., MOPA) configuration is used in which the oscillator utilizes a low power, core pumped Yb: silica laser followed by a cladding pumped amplifier. Such fiber lasers, when resonated with fiber Bragg gratings (hereafter, FBGs), can provide multi-watt powers with narrow line widths and excellent spatial mode quality. The output of lasers 101-103 are mixed with the output of a Nd : YAG laser 105 using several non-linear optical crystals 107 to provide the desired wavelengths. The output of Nd : YAG laser 105 is preferably fixed at 1319 nanometers although other wavelengths (e. g., 1064 and 946 nanometers) can also be used with the invention.

Similarly, other solid-state lasers can be mixed with the output of lasers 101-103 (e. g, Er: silica fiber systems).

In one embodiment, crystals 107 are LiNbO3 crystals. The output of this system can be increased to the tens of milliwatts level by replacing the non-critically phase matched bulk LiNbO3 crystals with periodically poled lithium niobate (hereafter, PPLN). Alternately, crystals 107 can be comprised of KDP, BBO, or other non-linear crystalline material.

Fig. 2 illustrates the expected tuning range of a frequency doubling and/or SFG system based on tunable Yb: silica and fixed 1319 nanometer output Nd : YAG lasers.

Due to the wide tuning range of the Yb : fiber lasers (i. e., approximately 1020 to 1200

nanometers), the tuning range of a frequency doubled system 201 is approximately 510 to 600 nanometers while the tuning range of a SFG system 203 is approximately 575 to 630 nanometers. Thus the combination of these two systems yields an overall tuning range of approximately 510 to 630 nanometers. Recognizably, performance is expected to be weak at the ends of the tuning ranges.

Laser 105 is preferably fixed at a wavelength of approximately 1319 nanometers. A suitable Nd : YAG laser is fabricated by Lightwave Electronics, model 126-1319-350. This laser is a tunable single mode laser with a thermal drift below 50 MHz/hour. Preferably lasers 101-103 are narrow band Yb : silica lasers. Alternately, lasers 101-103 can be a broadband Yb: silica laser, for example one with a bandwidth of approximately 4 nanometers, that is resonated to achieve the desired bandwidth.

Preferably this is done with a FBG, thereby affording some tunability via thermally- induced grating period changes. Fiber gratings are already available for a few standard wavelengths from New Focus as the model 5900 series. Installation of a fiber grating with the proper period would guarantee operation near the desired process wavelength, thus requiring only fine tuning. Ultra-narrow bandwidth lasers, for example lasers with sub-MHz linewidths, have already been fabricated.

Regarding non-linear optical elements 107, bulk LiNbO3 is routinely available and PPLN is becoming widely known. Although long term reliability has not yet been demonstrated, in the preferred embodiment of the invention PPLN is used due to its high non-linear coefficient, non-critical phase matching, and high bandwidth.

Regardless of whether LiNbO3 or PPLN elements are used, operation at high temperatures to achieve non-critical phase matching reduces the tendency toward photorefractive distortion of the laser waves.

The theoretical conversion efficiency for sum frequency generation with non-critical phase matching has been described with a well known formula quoted by Moosmuller et al. (Moosmuller et al., Sum-Freqzencv Generation of Continuous-Wave Sodium Da Resonance Radiation, Optics Letters, Vol. 22, No. 1135, 1135-1137 (August 1,1997)), which can be re-expressed as:

where the P's are the various powers at the three wavelengths 1-3, n3 is the crystal refractive index at the output wavelength, L is the crystal length, deff is the non-linear optical coefficient, and Zfs is the impedance of free space (i. e., 377 ohm). For typical input wavelengths of 1100 and 1300 nanometers, an output wavelength of 600 nanometers, and a refractive index of 2.2, P 2. 9xl0''d' LPP 3 ciT 1 2 Assuming an L and deter for bulk LiNbO3 of 50 millimeters and 3.3 pm/V, respectively, P3 is approximately 0.016 PIP,. For a 20 millimeter PPLN sample with a non-linearity of 18 pm/V, P3 is approximately 0.19 PlP2. Thus the conversion efficiency ranges from approximately 1.6 percent per watt to approximately 19 percent per watt. If the input powers are adjusted to provide a PIP2 of approximately 1 watt (e. g., 0. 35 watts from the 1319 nanometer Nd : YAG laser and 3.0 watts at approximately 1100 nanometers from the Yb: silica laser), approximately 1 milliwatt is obtained from the bulk crystal or approximately 190 milliwatts from the PPLN. Additionally, since the FWHM temperature tuning bandwidth is expected to be tenths of a degree centigrade, and since self-absorption is not serious, stable, hysteresis-free operation can be obtained.

Spectral purity of CW fiber lasers can be impressive, especially when the pump power is stabilized. Linewidths below 1 MHz are achieved, and values in the kHz have been reported. There are a number of reasons for this performance. First, there are no high frequency, transient, refractive index disturbances due to turbulent dye flow.

Second, since fiber gratings can be long and effectively contain many grooves, they can be spliced into thermally stable, vibration free positions. Third, rare earth ion energy storage lifetimes are on the order of milliseconds, preventing rapid changes in the laser inversion and reducing mode-beating effects. Fourth, in three-level lasers like the 1500 nanometer Er3+ laser or the 1100 nanometer Yb3+ laser, the standing laser wave bleaches its own perfectly resonant transmission grating in weakly pumped regions of the fiber.

This subtle effect, which is the opposite of the familiar spatial hole burning in a four-level laser, can prevent lasing at other wavelengths. If both of the input lasers operate in an ultra-narrowband mode, the SFG signal should be similarly pure.

Tuning and wavelength locking in the preferred embodiment is expected to be simple. The Yb : silica laser is outfitted with a fiber Bragg grating and is temperature tuned to achieve the larger wavelength excursions. Pressure tuning of fiber gratings has

also been demonstrated, with a high (i. e., kHz) bandwidth. It is possible to use a proportional integral differential sort of lock loop to control each of the lasers affecting a particular SFG output frequency.

A SFG-based embodiment of the invention offers a variety of benefits.

First, a single technology can be used to generate all three wavelengths necessary for isotope separation. Second, all of the major subassemblies are commercially available.

Third, the lasers of choice are compact, robust, and air cooled, thus eliminating the need for special facilities. Fourth, laser tuning can be accomplished without the need for moving parts. Fifth, the SFG approach can generate powers in the range of tens to hundreds of milliwatts. Sixth, CW operation eliminates timing concerns and simplifies wavelength control. Seventh, the limited tuning ranges of the Nd : YAG and fiber grating resonated Yb: silica lasers prevent wide wavelength excursions. Eighth, the invention can be easily integrated with standard fiber optical components. Ninth, the maintenance of this system is minimal in comparison to that required of a standard DMO.

The use of a true CW master oscillator based on ultra-narrowband lasers as presently disclosed allows simplification of some components of the WFGs.

Furthermore, given that the visible light signals are low power and near-diffraction limited, efficient fiber transport is possible, potentially allowing further savings in cost, spacer, and alignment time. Possible changes in the various WFG components are discussed further below as well as illustrated in Fig. 3.

The outputs of a Nd : YAG laser 301 and a Yb: silica laser 303 are mixed in a SFG, non-linear crystal (e. g., LiNbO3 or PPLN), a portion of which is reflected by a beam splitter 307. Since the SFG wavelength depends on two IR input wavelengths. there are two wavelengths to monitor and control. Both of them can be delivered with a single fiber 309, and separated with a standard telecommunications-type wavelength division multiplexer 311 prior to connection to a fiber optic switch 313. Suitable fiber optic switches are manufactured by JDS Fitel (e. g., the SB series) and by DiCon (e. g., the MC523 series). The output of fiber optic switch 313 is coupled to a wavemeter 315. A suitable wavemeter is the Burleigh WA-1500 which offers frequency measurement with 30 MHz accuracy and 10 MHz display resolution. Since this wavemeter contains a computer interface, accepts a user set-point, and delivers an analog feedback error signal, it can be easily integrated with the electronically tuned Nd : YAG and Yb: silica lasers.

The output of master oscillator 317 is passed to an electro-optic phase modulator 319 and an electro-optic amplitude modulator 321 in order to broaden the spectrum. Such modulators can be fabricated from LiTa03. Preferably modulators 319 and 321 are designed to work with high peak power, pulsed laser beams, and have millimeter-scale apertures, thus requiring RF amplifiers 323 to apply large RF drive voltages in order to obtain sufficient modulation indices. To prevent heating of modulators 319 and 321 and the resonant circuits, preferably the RF drive is not applied continuously but is instead pulsed synchronously with the light wave. Operation with low-power light at a small beam diameter would probably allow use of fiber pigtailed integrated-optic modulators based on LiNbO3 waveguides. Representative modulators are made by Uniphase Telecommunications Products (formerly United Technologies Photonics) and E-TEK Dynamics. If needed, extra bandwidth can be gained by putting two modulators in series.

In the preferred embodiment, one of the dye chains requires co- amplification. In other words, the two process wavelengths are amplified simultaneously within the same medium. To obtain the optimum power split between the two wavelengths, a power balance function is needed. Although this function can be provided with an arrangement of polarizers and a high voltage Pockels cell, preferably a low voltage, fiber pigtailed amplitude modulator is used. Preferably modulator 321 can also provide the blanking function, wherein one of the process wavelengths is interrupted, for diagnosis of the photocurrent amplitude. Furthermore, continuous, low frequency modulation of the power split can be used with lock-in photocurrent detection to assure dynamic maximization of separative work in spite of variations in vapor density, overall laser power, etc.

Fig. 4 is an illustration of an alternate embodiment of the invention. In this embodiment the output of a Yb: silica narrowband oscillator 401 is modulated by a phase modulator 403. Modulator 403 is driven by RF source 405. The output of modulator 403 passes through a cladding pumped fiber amplifier 407. The output of amplifier 407 and a Nd : YAG laser 409, preferably operating at 1319 nanometers, are mixed by non-linear crystal 411. Preferably crystal 411 is a PPLN crystal, although as noted above, other crystals can also be used. The advantage of this embodiment is that the modulator need only handle the low powers emitted by oscillator 401 instead of the sum frequency mixed

output as shown in Fig. 3. This same configuration can be used with other oscillators as well as other solid-state lasers.

As will be understood by those familiar with the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.