Patent Application
20100265973
A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.
There are many types and categories of tunable lasers, and one known type of laser tenability is known as single line tuning. Since no real laser is truly monochromatic, all lasers can emit light over some range of frequencies, known as the linewidth of the laser transition. In most lasers, this linewidth is quite narrow (for example, the 1064 nm wavelength transition of a Nd:YAG laser has a linewidth of approximately 120 GHz, corresponding to a 0.45 nm wavelength range). Tuning of the laser output across this range can be achieved by placing wavelength-selective optical elements into the laser's optical cavity, to provide selection of a particular longitudinal mode of the cavity.
One such wavelength-selective optical element is an etalon which comprises two substantially parallel, partially transmitting mirrors. Transmission through an etalon is generally low except for a series of peaks, which are approximately equally spaced at an interval known as the free spectral range (FSR) of the etalon. The centre wavelength of an etalon transmission peak can be varied by changing the optical distance between the etalon mirrors. It is necessary for the etalon FSR to be substantially larger than the desired tuning range of the laser, to ensure that only one of the etalon transmission peaks is within the desired tuning range. The bandwidth of the transmission peaks is also an important parameter for laser tuning, since bandwidth determines the loss seen by the modes adjacent to the lasing mode, which in turn determines the side mode suppression ratio (SMSR). Both the bandwidth and free spectral range of an etalon can be varied according to known design principles.
US Patent Application Publication No. US2005/0008045 A1 describes a tunable laser in which a tunable etalon is used as a mirror within an external semiconductor cavity. The etalon, which is used to tune the said cavity, is tunable by microelecromechanical means for controlling the optical space between the two parallel mirrors. However, this has the effect of tuning only the wavelength of the laser emission, which can result in mode hopping. By way of brief explanation, a laser cavity can only support certain modes of oscillation, which modes can be longitudinal and transverse. Mode hopping is simply the laser jumping between possible modes, and for longitudinal mode hopping the laser wavelength is effectively jumping. Mode hopping is undesirable in many applications since it introduces unwanted intensity noise.
We have now devised an improved tunable laser, in which at least some of the problems associated with known systems are alleviated.
In accordance with the present invention, there is provided a laser system comprising a laser source, a laser cavity and a wavelength discriminating structure for receiving an input from said laser source and generating a laser output, said system further comprising means for selectively changing the angle of incidence of said input on said wavelength discriminating structure so as to cause simultaneous corresponding changes in the length of said laser cavity and the frequency of said laser output.
The present invention, therefore, enables reliable tuning of laser emission without mode hopping because the wavelength discriminating structure (possibly a diffraction grating but preferably an etalon structure) is designed such that wavelength and cavity length are tuned simultaneously by selectively changing the angle of incidence of the laser input on said wavelength discriminating structure. First and second etalon mirrors may be provided on respective first and second substrates, which are then beneficially arranged and configured such that adjustment of said etalon structure relative to said laser source (preferably rotation of said etalon structure relative to said laser source about an axis which is transverse relative to the optical path of the laser system) causes a corresponding change in the angle of incidence of said laser input thereon, and more preferably, the first and second substrates and said respective first and second mirrors are configured to act as an effective, substantially shear plate.
In one preferred embodiment, said first substrate has an input surface and said second substrate has an output surface, said input and output surfaces being substantially parallel to each other and non-parallel to said first and second mirror. For example, said first and second substrates may comprise matched transmissive wedges.
The space between said first and second mirrors preferably comprises a hermetically sealed air gap, such that the structure is temperature insensitive.
The system preferably further comprises a filter for limiting the spectrum of said input from said laser source to a predetermined tuning range, which predetermined tuning range is beneficially substantially equal to the free spectral range (FSR) of said etalon structure.
These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.
Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:
a and 3b illustrate schematically the etalon structure provided in the system of
A tunable, narrow linewidth laser according to a preferred exemplary embodiment of the present invention is designed to be robust, with a linewidth of less than 500 kHz, frequency chirps over 100 GHz at a 1 kHz repetition rate, and no mode hopping.
Referring to
Laser output from the gain medium 10 passes through a collimating lens 16 to an infrared (IR) filter 18, arranged and configured to limit the optical spectrum to 100 GHz, i.e. the tuning range and FSR of the etalon 20 (as illustrated schematically by
The etalon 20 comprises two parallel, partially transmitting mirrors 22a, 22b provided on respective matched wedge plates 24a, 24b. The optical space between the reflectors 22a, 22b is defined by opposing spacers 26 provided therebetween. Thus, the spacing between the etalon reflectors 22a, 22b is fixed. Laser light output from the etalon 20 passes to a laser output coupler 28 (e.g. R>90%) to provide the desired chirped laser output 30.
Rotation of the etalon reflectors 22a, 22b enables the laser emission wavelength to be tuned through 100 GHz. In this exemplary embodiment of the invention, the etalon substrates are matched wedges 24a, 24b which create an effective, relatively thick shear plate which, when rotated, also change the cavity length due to the fact that the structure allows the angle of incidence (AOI) of the laser transmission to be altered, as will be explained in more detail later. In other words, etalon rotation tunes the laser emission wavelength and the cavity length simultaneously, thereby eliminating mode hopping and enabling reliable tuning of the laser emission through 100 GHz. Considering the illustrated exemplary embodiment, for the etalon to tune through 100 GHz, the effective etalon cavity length will need to change by 100e9/1e14, i.e. 1000 ppm. For elimination of mode hop during tuning, the following expressions must be satisfied:
Nλ/2=Lcavity (Laser condition)
mλ=2nd cos (Etalon condition)
When the laser frequency increases by 100 GHz (1000 ppm), the laser cavity will need to decrease in length by 1000 ppm*30 mm, i.e. 30 microns, in order to avoid mode hopping. By the use of an effective shear plate (via the matched wedges 24a, 24b) built into the etalon substrates 22a, 22b, when the etalon is rotated, the laser cavity length will change by the correct amount to allow mode hop free tuning.
Referring additionally to
As illustrated, upon rotation of the etalon structure 20, the angle of incidence θi of the input light 34 on the effective shear plate formed by the matched wedges 24a, 24b, as well as the angle of incidence on the etalon to be changed, which enables optimised path and frequency tracking to avoid mode hops and the need to provide several actuators. In more detail, etalon frequency transmission peaks can be tuned by means of rotation of the etalon structure in accordance with the following statement:
Δλ/λ=−2/2n2
where λ is the output light frequency, Δλ is the frequency shift, is the etalon angle and n is refractive index of the optical space 32 between the reflectors 22a, 22b which, in this case, is 1. Thus, an etalon angle
tuned from AOI θi of 0.5 degrees to {tilde over ( )}2 degrees at 1550 nm will be frequency shifted by 100 GHz or 0.8 nm. As the etalon is rotated, its resonant wavelength decreases, which requires a shorter cavity length in order to eliminate the possibility of a mode hop. Thus, when the AOI on the substrate changes through θi, the etalon angle
increases, i.e. the resonant wavelength of the etalon decreases, and the AOI on the second substrate 24b decreases, creating an effective reduction in optical path length, as illustrated schematically by
Referring to
With reference to the accuracy of frequency tuning, 1 ppm frequency accuracy (which equates to an etalon displacement resolution of 1.25 nms) requires highly linear tuning, highly reproducible tuning or, more preferably, means for accurately measuring frequency tuning during operation. Referring back to
It should be noted that the present invention is not restricted to the above-described embodiment and preferred embodiments may vary within the scope of the appended claims. The term “comprising”, when used in the specification including the claims, is intended to specify the presence of stated features, means, steps or components, but does not exclude the presence or addition of one or more other features, means, steps, components or groups thereof. Furthermore, the word “a” or “an” preceding an element in a claim does not exclude the presence of a plurality of such elements. Moreover, any reference sign does not limit the scope of the claims. The invention can be implemented by means of both hardware and software, and several “means” may be represented by the same item of hardware. Finally, the features of the invention, which features appear alone or in combination, can also be combined or separated so that a large number of variations and applications of the invention can be readily envisaged.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0724874.3 | Dec 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB2008/004253 | 12/22/2008 | WO | 00 | 6/17/2010 |
