Field of the Invention The present invention relates to a tuneable laser. Background of the Invention

Tuneable lasers, where the emission wavelength can be varied, are important in a number of applications, such as, telecommunications, spectroscopy and optical coherence tomography. One way to tune a laser is to insert a prism or grating inside the laser cavity which selects wavelengths that are allowed to recirculate around the cavity and be amplified. Other wavelengths are expelled from the cavity, so laser emission at these other wavelengths will be suppressed. However, prisms and gratings are fairly bulky, so they are not well suited to building compact lasers. Also, prisms and gratings tend to be relatively expensive, and aligning them is time consuming and complex.

Instead of using a prism or grating, an alternative is to use an interference filter. An interference filter can be designed to transmit a narrow band of selected wavelengths which are allowed to recirculate around the laser cavity, leading to laser emission which corresponds with the transmission wavelengths of the interference filter.

An interference filter is simpler to align and more compact than a prism or a grating. However, the designed transmission wavelengths of an interference filter are fixed, so it may be necessary to replace the interference filter if it is desired to make the laser operate at a different wavelength. Replacing the interference filter to tune the laser is not particularly convenient in applications, such as spectroscopy, where it is desirable to be able to adjust the laser wavelength during a measurement.

The designed transmission wavelengths of an interference filter usually assume that a laser beam will hit the interference filter at normal incidence. Rotating the interference filter so that the laser beam is not at normal incidence alters the transmission wavelengths of the interference filter, which makes limited wavelength tuning possible. Kischkat, J. et al describe rotating the angle of an interference filter to adjust the wavelength of a quantum cascade laser in "Alignment-stabilized interference filter-tuned external-cavity quantum cascade laser", Optics Letters, 1 December 2014, Vol. 39 No. 23, 6561.

However, rotating an interference filter can only provide limited wavelength tuning to wavelengths shorter than the designed transmission wavelengths of the interference filter for normal incidence. Additionally, rotating an interference filter has the disadvantage that at rotation angles where the interference filter is near normal with the laser beam, the rejected wavelengths tend to be reflected back into the gain medium meaning that the rejected wavelengths can be amplified leading to the laser emission spectrum containing unwanted wavelengths. Furthermore, as the angle of incidence between the interference filter and the laser beam increases, transmittance through the interference filter is reduced (resulting in reduced feedback into the gain medium) and the transmission bandwidth of the filter is increased (leading to unwanted wavelengths in the laser emission spectrum). It would, therefore, be desirable to find an improved way to tune a laser which allows a compact laser system to be built, but which does not have the disadvantages of using a rotating interference filter.

Summary of the Invention

According to a first aspect of the invention, there is provided a tuneable laser. The tuneable laser has a gain medium, a cavity mirror, an interference filter and a positioning mechanism. The gain medium has an emission spectrum and the gain medium is configured to emit a laser beam. An interference filter is located between the gain medium and the cavity mirror. The interference filter is configured to transmit a portion of the emission spectrum in order to suppress laser emission by the gain medium at wavelengths other than the transmitted portion. The portion of the emission spectrum transmitted by the interference filter varies across the interference filter. The positioning mechanism is configured to move the interference filter relative to the laser beam in order to select the portion of the emission spectrum transmitted by the interference filter, thereby determining the emission spectrum of the tuneable laser.

This arrangement has a number of advantages over a tuneable laser using a prism or grating, including being more compact (making it well suited to incorporating into small laser packages), being simpler to align, and being cheaper than gratings or prisms. Compared to tuning a laser using a rotating interference filter, this arrangement does not suffer from the problem of unwanted wavelengths being reflected back into the gain medium, because the angle of the interference filter need never be close to normal to the laser beam.

Also, the positioning mechanism does not need the fine level of control required with a rotating interference filter (where small adjustment can lead to significant wavelength tuning). Instead, the interference filter may have a coarse, or stepwise, transition in wavelength across the interference filter, so that small changes in position of the interference filter (for example, from a vibration) do not have an effect on the laser emission spectrum. As the laser is less susceptible to movements in the position of the interference filter, a less precise (and therefore cheaper) positioning mechanism may be used. Also, the laser emission spectrum may be less susceptible to movement caused by shrinkage during curing of adhesive or potting compound (used, for example, to fix optical elements in a laser package).

Unlike using a rotating interference filter, where the amount of wavelength tuning is limited, using an interference filter where the portion of the emission spectrum transmitted by the interference filter varies across the interference filter may provide as much wavelength tuning range as required for a particular gain medium. The wavelength transition across the interference filter may vary in any desired pattern. For example, a gradual transition to allow small adjustments, a stepwise transition to allow for rapid and significant changes in wavelength, a transition that increases then decreases, or a transition that follows a pattern of wavelengths required for a particular experiment or measurement. Another advantage over a rotating interference filter, where the transmittance and transmission bandwidth varies uncontrollably as the interference filter is rotated, is that this arrangement provides complete control over the transmittance and transmission bandwidth, and a much wider wavelength tuning range. This makes the tuneable laser a useful source for applications (such as optical coherence tomography) which require a laser with both a narrow emission spectrum and a wide wavelength tuning range.

The interference filter may be arranged at an oblique angle to the laser beam in order to prevent reflected laser beam, containing unwanted wavelengths, from striking the gain medium. This prevents the unwanted wavelengths that are reflected by the interference filter from being amplified by the gain medium, leading to laser output which is not contaminated by these unwanted wavelengths.

The interference filter may be tilted with respect to the laser beam so that unwanted wavelengths are reflected away from the gain medium but movement of the interference filter by the positioning mechanism does not change the distance between the interference filter and the gain medium. Keeping the distance between the interference filter and the gain medium as consistent as possible has been found to improve the coupling of the laser beam from the cavity back into the gain medium, leading to a more stable and reliable laser, with reduced intensity fluctuations, across the full wavelength tuning range. As the positioning mechanism moves the interference filter relative to the laser beam, the laser beam may scan across a region of the interference filter. The region may be generally aligned with the direction of movement of the interference filter. The positioning mechanism may translate the interference filter across the laser beam. Translating the interference filter may scan the laser beam across the region of the interference filter. The region may be arranged generally parallel with the direction of translation of the interference filter.

The interference filter and the cavity mirror may be formed as a single optical element. This reduces the number of optical components required to build the laser, making the laser cheaper and easier to align because there are less degrees of freedom, which is a particular advantage for building laser packages. It has not been possible to combine the interference filter and cavity mirror into a single optical element in tuneable lasers which tune the laser by rotating an interference filter, because rotating a single optical element combining the interference filter and the cavity mirror to tune the transmission wavelength of the interference filter would also misalign the cavity mirror. This is not a problem in the present invention because translating a single optical element to tune the wavelength does not misalign the cavity mirror.

The interference filter may be deposited on the cavity mirror.

The tuneable laser may further comprise an optical element having a first surface comprising the cavity mirror and a second surface comprising the interference filter. The second surface may be at an oblique angle with respect to the first surface. This provides a compact and easy to align arrangement, which simultaneously prevents unwanted wavelengths from being amplified by the gain medium. The compact nature of this arrangement makes it particularly suited to incorporating into a laser package. Moving the interference filter may comprise translating the interference filter. The positioning mechanism may comprise a translation stage configured to translate the interference filter across the laser beam. The translation stage may comprise a piezoelectric actuator, which allows fast but accurate scanning of the emission wavelength, for example, in applications such as optical coherence tomography where a narrow and accurate laser emission spectrum must be scanned quickly over a wide wavelength tuning range.

The tuneable laser may further comprise a spectrometer and a motion controller. The spectrometer may be configured to determine the laser emission spectrum. The motion controller may be configured to control the translation stage, based on the determined spectrum, in order to obtain a desired laser emission spectrum.

The interference filter may comprise a linear variable filter. The linear variable filter may comprise one of: a linear variable band pass filter; a linear variable edge pass filter; or a linear variable longpass edge filter in combination with a linear variable shortpass edge filter.

The laser may be a mid-infrared laser. The laser may comprise a semiconductor gain medium having a mid-infrared emission spectrum. The mid-infrared emission spectrum may be in the range of 2.5 micrometers and 20 micrometers.

The laser may be a cascade laser. For example, the laser may be one of: a quantum cascade laser; or an inter-band cascade laser. Cascade lasers are useful sources in the mid-infrared, for example, for gas sensing, spectroscopy and optical coherence tomography. The emission spectrum of cascade lasers is controlled by thickness of epitaxial layers of the cascade laser, offering freedom to obtain a desired emission spectrum which suits a particular application by adjusting the thickness of one or more epitaxial layers. However, small variations in the manufacturing process can result in unexpected variations in the composition and thickness of the layers, leading to a cascade laser which does not have the desired emission spectrum. This results in low yields, high waste and therefore high prices. The present invention can be used to adjust the emission spectrum of a given cascade laser, so less fabricated cascade lasers are wasted, reducing the cost to manufacture cascade lasers and therefore the price to purchase a cascade laser, making cascade lasers viable for new applications. A lens may be located between the gain medium and the interference filter. The laser beam may diverge from the gain medium and the lens may collimate the laser beam before the interference filter and refocus the laser beam reflected from the cavity mirror back into the gain medium. The lens may be a ball lens (which are economical), or an aspheric lens (which may provide better coupling of the laser beam back into the gain medium and may be thinner than a ball lens). The lens may be antireflection coated to reduce optical losses in the external cavity of the laser and to enhance the suppression of laser emission by the gain medium at wavelengths other than the transmitted portion.

The tuneable laser may comprise a plurality of gain media. Each gain medium of the plurality of gain media may have an emission spectrum and may be configured to emit a laser beam. The interference filter may be configured to intercept the laser beam emitted from all of the gain media. For each gain medium, the interference filter may be configured to transmit a portion of the emission spectrum in order to suppress laser emission by the relevant gain medium at wavelengths other than the transmitted portion. The positioning mechanism may be configured to move the interference filter relative to the laser beam in order to select the portion of each of the emission spectra transmitted by the interference filter, thereby determining the emission spectra of the plurality of gain media. This allows the emission wavelength of several gain media to be tuned simultaneously, using a single interference filter. According to a second aspect of the invention, there is provided a method of tuning a laser according to the first aspect.

The method may comprise locating an interference filter between a gain medium and a cavity mirror, wherein the interference filter transmits a portion of the emission spectrum in order to suppress laser emission by the gain medium at wavelengths other than the transmitted portion, and wherein the portion of the emission spectrum transmitted by the interference filter varies across the interference filter. The method may comprise translating the interference filter relative to the laser beam in order to select the portion of the emission spectrum transmitted by the interference filter, thereby determining the emission spectrum of the tuneable laser.

The method may further comprise placing the interference filter, gain medium and cavity mirror in a package. The method may further comprise translating the interference filter with a removable translation mechanism to tune the laser, fixing the alignment of the interference filter with respect to the gain medium and the cavity mirror in the package, and removing the translation mechanism. This reduces the cost and size of the laser as it is not necessary to include a translation stage in the package, providing a compact and economical wavelength-specific laser

The alignment may be fixed using one of: a mechanical fastener, an adhesive and a potting compound.

Brief Description of the Drawings

The invention shall now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates a tuneable laser according to an embodiment of the invention, where the tuneable laser has been adjusted to emit a first wavelength.

Figure 2 illustrates the tuneable laser of Figure 1 adjusted to emit a second wavelength;

Figure 3 illustrates a side view of an alternative arrangement for the linear variable filter in the tuneable laser of Figure 1;

Figure 4 illustrates a tuneable laser according to an embodiment of the invention, where the interference filter and high reflector cavity mirror are combined into a single optical element; and

Figure 5 illustrates a tuneable semiconductor laser package according to an embodiment of the invention.

Detailed Description

Figure 1 illustrates a tuneable laser 100. The tuneable laser 100 has a cavity defined by two cavity mirrors - high reflector 120 and output coupler 122. The tuneable laser 100 also has a gain medium 110 with a broad gain bandwidth, so the gain medium 110 is capable of emitting a laser beam 111 having a spectrum with a number of wavelength components 112a, 112b and 112c.

A linear variable filter 130 is placed between the gain medium 110 and the high reflector 120. The linear variable filter 130 transmits a narrow range of wavelengths, with the specific range of wavelengths transmitted varying in the direction 131 across the linear variable filter 130. In the position shown in Figure 1, the linear variable filter 130 transmits wavelength 112b. The transmitted wavelength 112b is reflected back and forth between the high reflector 120 and the output coupler 122, traveling through the gain medium 110 where wavelength 112b is amplified. In contrast, wavelengths 112a and 112c are reflected by the linear variable filter 130. The linear variable filter 130 is angled such that the reflected wavelengths 112a and 112c are reflected out of the laser cavity, ensuring that they do not strike the gain medium 110 to make sure that they are not amplified by the gain medium 110.

The laser output 160, which leaks through the output coupler 122, will be predominantly wavelength 112b which has been amplified, whereas wavelengths 112a and 112c will be suppressed.

Figure 2 illustrates how to adjust the emission spectrum of the tuneable laser 100. The linear variable filter 130 is on a translation stage 140. The translation stage 140 is moved in the direction 142 so that the laser beam 111 strikes a different part of the linear variable filter 130 which transmits wavelength 112a, but reflects wavelengths 112b and 112c. As a result, the laser output 160 will now be shifted to wavelength 112a, and emission at wavelengths 112b and 112c will be suppressed.

Figures 3 illustrates a side view of an alternative arrangement for the linear variable filter 130. In this example, the linear variable filter 130 is aligned so that movement of the linear variable filter 130 to adjust the emission spectrum of the laser 200 does not vary the distance 216 between the gain medium 110 and a region 215 of the linear variable filter 130 that the laser beam 111 hits as the linear variable filter 130 is translated. The linear variable filter 130 is attached to the top of translation stage 140 by angled mount 232. Angled mount 232 has a central aperture which allows transmitted wavelength 112b to pass through the angled mounted 232. The angled mount 232 tilts the linear variable filter 130 with respect to the laser beam 111 so that the reflected wavelengths 112a and 112c are reflected away from the gain medium 110. The linear variable filter 130 is arranged so that the wavelengths transmitted by the linear variable filter 130 vary in direction 131 (orthogonal to the plane of the drawing). The emission wavelength of the laser 200 may be varied by moving the linear variable filter 130 in direction 131 using the translation stage 140, which scans the laser beam 111 across the region 215. The linear variable filter 130 is aligned on the translation stage 140 so that the direction 142 (orthogonal to the plane of the drawing) in which the translation stage 140 is translated is generally parallel with the region 215 of the linear variable filter 130, so that as the translation stage 140 is translated in direction 142, the distance 216 between the region 215 of the linear variable filter 130 and the gain medium 110 does not vary. The arrangement shown in Figures 2 to 4 can be made more compact by combining the linear variable filter 130 and the high reflector 120 into a single optical element. One way to do this is to coat the outer surface of the high reflector 120 with the linear variable filter.

Figure 4 shows a tuneable laser 300 which illustrates another way to combine the linear variable filter 130 and the high reflector 120 into a single, wedge-shaped, optical element 270. The arrangement in Figure 4 is not only compact, but also has the added advantage of simultaneously ensuring the unwanted wavelengths (in this case 112a and 112c) are reflected out of the laser cavity away from the gain medium 110.

The single optical element 270 has a front face 230 coated with a linear variable filter. In this position the linear variable filter material transmits wavelength 112b and reflects wavelengths 112a and 112c. The single optical element 270 is coupled to translation stage 140. By moving the translation stage 140 (as described in relation to Figures 1 and 2), the wavelength transmitted by the linear variable filter can be selected. The front face 230 is angled with respect to the laser beam 111, so that the reflected wavelengths 112a and 112c do not hit the gain medium 110.

The rear face 220 acts as the high reflector cavity mirror, reflecting the transmitted wavelength 112b around the cavity. So, the rear face 220 is coated with a material that is highly reflective across the full gain bandwidth of the gain medium 110 and the rear face 220 is normal to the laser beam 111 so as to retroreflect the laser beam 111.

Instead of the arrangement shown in Figure 4, the single optical element 270 and translation stage 140 could be arranged as shown in Figure 3.

The technique for wavelength tuning a laser described above is suitable for use with any gain medium, such as, a solid state, gas, or semiconductor gain medium.

The technique described in Figures 1-4 is particularly suitable for manufacturing a compact tuneable external cavity semiconductor laser. For example, a cascade laser, such as, a quantum cascade laser or an inter-band cascade laser. Cascade lasers are useful sources in the mid-infrared for gas sensing, spectroscopy and optical coherence tomography. The emission spectrum of cascade lasers is controlled by thickness of epitaxial layers of the laser, offering freedom to select a desired emission spectrum which suits a particular application by adjusting the thickness of one or more epitaxial layers. However, small variations in the manufacturing process can result in unexpected variations in the composition and thickness of the layers, leading to a cascade laser which does not have the desired emission spectrum. This results in low yields, high waste and therefore high prices. The technique described in Figures 1-4 can be used to manufacture cascade lasers with a well-defined emission spectrum.

Figure 5 shows an example of a tuneable semiconductor laser 400. The tuneable semiconductor laser 400 is housed in package 305 which contains a semiconductor gain medium 315, such as a cascade laser gain medium.

The rear facet 316 of the gain medium 315 has been antireflection coated, so that laser beam 111 is coupled into the external cavity 318. The laser beam 111 diverges from the rear fact 316, so a lens 319 is located after the rear facet 316 to collimate the laser beam 111. The lens 319 is anti-reflection coated to reduce losses in the external cavity 318 and to improve suppression of wavelengths 112a and 112c.

A linear variable filter 130 is located between the lens 319 and the high reflector cavity mirror 120. The translation of the linear variable filter 130 is adjusted using translation stage 140 in the direction 142 to select the desired wavelength of the laser emission 160 output through aperture 306 (in the same way as Figure 1 and 2). Alternatively, the linear variable filter 130 and the translation stage 140 could be arranged as shown in Figure 3.

Once the desired emission spectrum of the laser 400 has been attained, the variable linear filter 130 may be locked in place (either mechanically, or using an adhesive or potting compound such as epoxy) and the translation stage 140 may be removed at this stage which reduces the cost and size of the laser 400 as it is not necessary to include a translation stage 140 in the package 305, providing a compact and economical wavelength-specific laser.

Alternatively, the translation stage 140 may be left inside the package 305 to allow for future adjustment of the emission spectrum of the laser 400. Being able to adjust the emission spectrum of the laser 400 is useful in some applications (such as spectroscopy, gas sensing and optical coherence tomography) where it is desirable to select, or scan, the emission spectrum to suit a particular experiment or measurement. The translation stage 140 may be electronically actuated, to make it easier to control the wavelength of the laser 400 without needing to open package 305, which may be sealed to prevent contamination or damage of the optical components. The translation stage 140 may be driven by a piezo actuator, which would allow fast and accurate scanning of the emission spectrum. The tuneable semiconductor laser 400 provides a useful source for applications (such as optical coherence tomography) which require a narrow emission spectrum to be selected at the same time as allowing scanning across a wide wavelength range.

A spectrometer may be incorporated into the package 305 to monitor the output of the laser (for example, by monitoring the unwanted reflections 112a and 112c), which may be used as a feedback to control the translation stage 140 to drive translation stage 140 to select a desired emission spectrum for the laser 400.

Instead of the separate interference filter 130 and high reflector 120 shown in Figure 5, the laser 400 could equally use the single optical element 270 shown in Figure 4, which would advantageously reduce the number of components, making the laser 400 cheaper to manufacture and making alignment easier by reducing the number of degrees of freedom to align.

Although the invention has been described in terms of a linear variable interference filter 130, the transmission wavelength need not vary linearly across the interference filter and may vary in other ways to allow appropriate wavelengths to be selected. The transmission wavelength may vary across the interference filter in any pattern of wavelengths required for a particular experiment or measurement. For example, the transmission wavelengths may vary in a stepwise fashion, in order that the laser emission wavelength may be rapidly changed between desired wavelengths by translating the interference filter between steps. The transmission wavelengths may vary gradually across the interference filter to allow small adjustments of the laser emission wavelength to be made. The wavelength may increase then decrease, or decrease then increase, across the interference filter. The tuneable lasers 100, 200, 300 and 400 could have multiple gain media 110 arranged in a side-by- side arrangement along direction 142 in front of the interference filter 130. The laser beam 111 emitted by each gain medium 110 passes through the interference filter 130. The selected wavelengths are transmitted by the interference filter 130 and reflected by the high reflector cavity mirror 120 back into the respective gain medium 110, while the rejected wavelengths are reflected by the interference filter 130 out of the cavity. The laser beam 111 emitted by each gain medium 110 intercepts a different portion of the interference filter 130 and the wavelength transmitted by that portion of the interference filter 130 controls the emission spectrum of that particular gain medium 110. By moving the interference filter 130, the emission wavelength of all of the gain media 110 can be controlled simultaneously.