Optical: systems and elements – Optical frequency converter – Parametric oscillator
Reexamination Certificate
2001-05-22
2004-03-23
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Parametric oscillator
Reexamination Certificate
active
06710914
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to tunable light sources, and more particularly to using the process of optical parametric oscillation (OPO) near degeneracy to obtain a light source with a wide and stable tuning range.
BACKGROUND OF THE INVENTION
The continuing optics revolution is bringing about changes in many fields of technology. For example, fiber-optic networks employing dense wavelength division multiplexing (DWDM) are becoming increasingly pervasive as the backbone of modern communications systems. At the same time, machining devices employing lasers for precision processing, e.g., cutting, scribing and/or polishing of various materials including biological tissue are displacing traditional mechanical equipment. In still other fields, laser-based systems are being adapted for display purposes.
The above-mentioned technologies, as well as many others, require light sources with appropriate performance parameters. Specifically, there is a demand for tunable light sources, i.e., tunable lasers that can be tuned over a wide range of wavelengths. Such tunable light sources should additionally exhibit excellent spectral characteristics, e.g., clean and narrowband output as well as absence of mode hops and/or power fluctuations during the tuning process. Furthermore, suitable light sources need to be simple in construction, versatile, and economical.
Such tunable laser sources are desired, for instance, in swept wavelength testing of passive and active telecommunication components. Testing a component can include, for example, measuring transmission, reflection or loss for any combination of the component's ports as a function of wavelength. Swept wavelength testing requires a very wide tuning range and/or a narrow test beam spectrum. In some cases a tuning range of 250 nm with a 0.1 to 10 pm test signal bandwidth is required. In addition to the swept wavelength approach, optical component testing can also be performed by a step-and-measure approach, by measurements at discrete wavelengths, or other variants of combining the tuning properties of the laser with measurements of relevant data. In this document, the term swept wavelength testing is intended to include these variants. Tunable laser sources are also employed in multi-channel coherent communication systems, spectroscopic measurements, and optical amplifier characterizations.
The prior art teaches the use of extended (or external) cavity diode lasers (ECDLs) to provide tunable laser sources for swept wavelength testing in telecommunications and other applications. A detailed description of external cavities is well documented in the art, for example, in “Spectrally Narrow Pulsed Dye Laser without Beam Expander” by Littman et al., Applied Optics, Vol. 17, No. 14, pp. 2224-2227, Jul. 15, 1978; “Novel geometry for single-mode scanning of tunable lasers” by Littman et al., Optics Letters, Vol. 6, No. 3, pp. 117-118; “External-Cavity diode laser using a grazing-incidence diffraction grating” by Harvey et al., Optics Letters, Vol. 16, No. 12, pp. 910-912; and “Widely Tunable External Cavity Diode Lasers” by Day et al., SPIE, Vol. 2378, pp. 35-41.
In a tunable ECDL the wavelength range is determined by the gain bandwidth of the lasing medium while wavelength selection and tuning functions are external to the gain element. These functions are typically accomplished by adjusting a total optical length L of the external cavity and its spectral response or passband. A diffraction grating and a movable mirror can be used for these purposes. The number of nodal points of the standing wave in the laser cavity is proportional to L/&lgr;, where &lgr; is the operating wavelength and L is the total optical length of the laser cavity (primarily provided by the length L
ext
of the external cavity) Therefore, if the wavelength tuning takes place while L is maintained constant, the number of nodal points in the laser cavity changes discontinuously. That is, the wavelength cannot be continuously varied, but rather, it leaps in discrete steps—termed as mode-hops. As a result, it is often difficult to tune in a desired wavelength, and there may also be substantial fluctuations in the output power of the laser.
The prior art teaches to mitigate or avoid mode-hops by varying the length L of the laser cavity as wavelength tuning is taking place. Coordinating the wavelength tuning and the cavity-length change in ECDLs has been a rather arduous and expensive undertaking. Documentation of further efforts to prevent mode-hops and provide more continuous tuning are found in U.S. Pat. Nos. 5,172,390, 5,319,668, 5,347,527, 5,491,714, 5,493,575, 5,594,744, 5,862,162, 5,867,512, 6,026,100, 6,038,239, and 6,115,401.
Diode lasers typically have gain bandwidths (and therefore tuning ranges) of about 1-5% of the optical wavelength, or about 30 nm if centered near 1550 nm. Some diode lasers which are optimized for broad gain bandwidth (at the expense of other properties) can have somewhat larger gain bandwidths. Therefore, external cavity diode lasers with tuning ranges of about 50-100 nm are now commercially available. However, tuning ranges approaching 250 nm are extremely difficult or impossible to achieve with a diode laser despite all the efforts documented in the prior art.
In U.S. Pat. No. 6,134,250 the inventors describe a single-mode wavelength selectable ring laser, which operates at a single wavelength selectable from any channel passband of a multiple-channel wavelength multiplex/demultiplex element (e.g., an arrayed waveguide grating router (AWGR)). A Fabry-Perot semiconductor optical amplifier (FP-SOA) is connected to AWGR to form a ring laser structure, where FP-SOA is used as an intra-cavity narrow-band mode-selecting filter to stabilize the laser oscillation to a single axial mode. As such, this ring laser system can only provide discrete tuning from one wavelength passband of the wavelength filter to another. That is, continuous tuning cannot be achieved in this system. Hence, this prior art laser system is suited for providing a wavelength-selectable laser, as opposed to a wavelength tunable laser.
Prior art also suggests turning to other types of lasers and elements to achieve a wide and stable wavelength tuning range. Unfortunately, none of the prior art systems has the desired parameters. Specifically, the gain bandwidths for the most promising of these lasers are limited, e.g., Erbium based lasers have gain bandwidths of about 30 nm to about 100 nm, SOA has a gain bandwidth of about 30 nm and ECDLs have gain bandwidths of about 100 nm. These gain bandwidths make it impossible to provide for tuning ranges up to 250 nm or more. Furthermore, these laser sources are not sufficiently simple in construction, versatile, and economical. Combining a number of them, e.g., stitching together several ECDLs to cover a tuning range of 250 nm, is not a practical solution. This is because it is difficult to control the tuning behavior or achieve accurate wavelength control of combined sources. Furthermore, combined sources can not be tuned as rapidly as some applications require. Also, an implementation including a combination of multiple sources is generally more expensive relative to a single source which covers the required wavelength range.
In order to generate light in certain wavelength ranges where laser sources are not available (e.g., due to lack of lasing media generating light in those wavelength ranges at sufficient power levels) the prior art prescribes the use of nonlinear optics methods. Nonlinear optics encompass various processes by which a nonlinear optical material exhibiting a certain nonlinear susceptibility converts input light at an input wavelength to output light at an output wavelength in the difficult to access wavelength range. Some well-known nonlinear processes involving light at two or more wavelengths (e.g., three-wave mixing and four-wave mixing) include second harmonic and higher harmonic generation, difference frequency generation, sum frequency generation and optical parametric
Arbore Mark A.
Myers Lawrence E.
Lee John D.
Lightwave Electronics
Lumen Intellectual Property Services Inc.
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