GB2569395A

GB2569395A - Mid-infrared cascade laser

Info

Publication number: GB2569395A
Application number: GB1721193.9A
Authority: GB
Inventors: Leslie Kennedy Kenneth; Revin Dmitry
Original assignee: Stratium Ltd
Current assignee: Stratium Ltd
Priority date: 2017-12-18
Filing date: 2017-12-18
Publication date: 2019-06-19
Also published as: GB2569456A; GB201721193D0; GB201820646D0

Abstract

Mid-infrared (MIR) cascade laser 100 comprises a gain medium 110 and cavity 130, 134, 136 to generate a set of longitudinal resonator modes. The cavity 130, 134, 136 comprises a first and second optical filtering element, wherein the first element selects a first subset from the set of modes and the second element selects a second subset of modes from the first subset. Method of tuning the laser 100, wherein the first and second elements are aligned in the cavity 130, 134, 136. Method of gas detection using the laser 100, wherein the set of modes includes a wavelength of an absorption line of a gas to be detected, the first subset includes said wavelength and the second element is adjusted to tune the second subset of modes to overlap with the wavelength of the absorption line. The first and second elements may be a bandpass filter and etalon respectively.

Description

Mid-infrared Cascade Laser

Field of the Invention

The invention relates to a mid-infrared cascade laser, a method of tuning a mid-infrared cascade laser and a method of gas detection using a mid-infrared cascade laser.

Background of the Invention

The mid-infrared region between around 2 μιτι and 20 μιτι is very useful for applications such as high resolution spectroscopy and gas analysis, since many strong gas absorption lines are concentrated in the mid-infrared region. Therefore, compact single mode lasers operating at room temperature in the mid-infrared region are desirable for targeting specific gas absorption lines for selective gas detection and analysis.

The most promising candidates for compact lasers in the mid-infrared wavelength range are cascade lasers, namely quantum cascade lasers and interband cascade lasers. The emission spectrum of a cascade laser is controlled by the thickness of epitaxial layers in the cascade laser. Therefore, in principal, it is possible to design a cascade laser with an emission spectrum suited to a particular application. However, in practice, cascade lasers with specific emission spectra are hard to grow because small variations in composition and thickness of the epitaxial layers, even down to the level of variations in a single monolayer, can alter the emission spectrum of a cascade laser. Also, it is difficult to manufacture a cascade laser which targets emission to a specific single mode.

Several ways to obtain, and tune, single mode emission from a broad spectrum cascade laser have been developed. For example, a dispersive optical element such as a diffraction grating can be inserted into an external laser cavity of the cascade laser, in order to select which emission wavelength is amplified. However, using a diffraction grating leads to a bulky and complicated cascade laser, which is highly sensitive to environmental conditions and is unsuited to operating outside a laboratory environment. Miniaturised single mode cascade lasers using an external cavity diffraction grating have tended to be unable to select narrow emission peaks because the selected peak width is related to the number of grating grooves illuminated by the laser beam, which is reduced for small diameter beams required in a miniaturised laser, particularly in the mid-infrared where diffraction gratings have a relatively low groove density.

-2Alternatively, a distributed feedback grating can be formed directly on top of, or below, an individual cascade laser waveguide structure, to cause the laser to emit a single longitudinal mode with some limited tuning possible through controlling the temperature and/or drive current of the laser. However, despite distributed feedback (DFB) cascade lasers being more compact, and more efficient at achieving single mode operation, than external cavity cascade lasers with a diffraction grating in the external cavity, DFB cascade lasers tend to be more expensive because DFB cascade lasers are challenging to manufacture. Any non-uniformity in the epitaxial layer thicknesses across the semiconductor wafer, and any slight variation in the manufacturing process during either etching or grating formation leads to a change in the effective refractive index for the emitting optical mode and consequently a change in the emission wavelength. As a result, yields are low, particularly when targeting a specific single mode, leading to high prices for single mode DFB cascade lasers.

An alternative method which has been suggested to produce single mode emission from a cascade laser has been proposed by Kischkat et. al. (Ultra-narrow angle tuneable Fabry-Perot bandpass interference filter for use as tuning element in infrared lasers, Infrared Science & Technology, 67(2014), 432-435). They propose incorporating a narrow bandpass mid-infrared interference filter, with a narrow transmission peak (less than 0.2 % of the central laser emission wavelength) into an external cavity quantum cascade laser system. The interference filter greatly simplifies the alignment and control of an external cavity cascade laser when compared to a diffraction grating and, when compared to a DFB cascade laser, far more of the cascade lasers that are produced can be tuned to a useful wavelength, increasing yield and reducing cost. However, although the authors note that this method provides a good candidate to produce single mode operation, the authors have been unable to demonstrate single mode emission using their method.

It would therefore be desirable to find a way to manufacture a single mode (or at least narrow bandwidth) cascade laser without the complexity of a diffraction grating or the cost of the DFB process.

Summary of the Invention

According to a first aspect of the invention, there is provided a mid-infrared cascade laser. The midinfrared cascade laser comprises a cascade gain medium and a cavity configured to generate a set of longitudinal resonator modes, the set comprising a plurality of longitudinal resonator modes of the cavity. The cavity comprises a first optical filtering element and a second optical filtering element.

-3The first optical filtering element selects a first subset from the set of longitudinal resonator modes. The second optical filtering element selects a second subset of longitudinal resonator modes from the first subset.

The present invention allows for a cascade gain medium, which has an emission spectrum which is close to a desired emission wavelength for a particular application but not quite ideal (for example, too broad) to be adjusted until the emission wavelength is more suited to the particular application. In this way, less fabricated cascade lasers with less than ideal emission wavelengths are wasted, reducing the cost to manufacture cascade lasers and therefore the price to purchase a cascade laser. This makes cascade lasers viable for new applications, such as gas sensing. The present invention also provides an effective way to tune a cascade laser for single mode operation, which is cheaper and easier than DFB, and more compact and reliable than using an external cavity diffraction grating.

The set of longitudinal resonator modes may contain more longitudinal modes than the first subset of longitudinal resonator modes. The first subset of longitudinal resonator modes may contain more longitudinal resonator modes than the second subset of longitudinal resonator modes. The second subset of longitudinal resonator modes may comprise a single longitudinal resonator mode.

The first optical filtering element may be a bandpass filter, such as a bandpass interference filter. The second optical filtering element may be an etalon. Relying on a bandpass filter alone (as in Kischkat) to select a single longitudinal mode may have problems, since narrow bandpass interference filters suitable to select a single mode tend to have low transmission efficiency (typically around 50% or less) in the mid-infrared. Such low transmission efficiency may reduce feedback from the external cavity, which may reduce the suppression of rejected wavelengths and result either in unstable single mode operation or more likely multi-mode operation. The suppression of rejected wavelength may improve as the transmission efficiency of the bandpass filter is increased, and mid-infrared bandpass interference filters with a much higher transmission efficiency may be more easily produced but only with broader transmission bandwidths which are not narrow enough to select a single longitudinal mode. So, a bandpass filter may be used in combination with an etalon to select a single mode. The bandpass filter may select a first subset of the plurality of longitudinal modes emitted by the gain medium, supressing laser emission at the wavelengths that are rejected by the bandpass filter, while the etalon selects a narrower second subset of the plurality of longitudinal modes selected by the interference filter, potentially selecting only a single mode.

-4The bandpass filter may have a transmission efficiency of one of: greater than 50 %; greater than 60 %; greater than 70 %; greater than 75 %; greater than 80 %; greater than 85 %; greater than 90%; and greater than 95%.

The bandpass filter may be tuneable. The tuneable bandpass filter may comprise one of: an angle tuneable bandpass interference filter and a variable linear filter.

The etalon may have an effective thickness such that the free spectral range of the etalon is wider than the first subset of longitudinal modes.

The transmission bandwidth of the bandpass filter is one of: greater than 0.5%; and greater than 1%.

The mid-infrared wavelength emitted by the mid-infrared cascade laser may be in the range of 2 pm and 20 pm.

The cascade gain medium may comprise a quantum cascade gain medium or an interband cascade gain medium.

The mid-infrared cascade laser may comprise a device configured to change the effective length of the cavity. Adjusting the effective length of the cavity may provide fine wavelength tuning, for example, to allow the emission spectrum of the cascade laser to be tuned to a particular gas absorption line. The device to change the effective length of the cavity may be an actuator configured to move a cavity mirror defining the cavity and/or a temperature controller configured to adjust a temperature of the cascade laser to change the effective length of the cavity.

The cascade gain medium may have a cavity facing facet that is coated with an anti-reflection coating having a residual reflectivity of less than 0.1 %, or less than 0.01%. Achieving a reflectivity of 0.1 % or less on the cavity facing (rear) facet improves stability of the cascade laser, by encouraging the laser to operate on modes of a combined cavity (incorporating the external cavity and the gain medium cavity), rather than the less stable modes of either the external cavity or gain medium cavity operating individually.

-5The cavity facing (rear) facet is at an oblique angle with respect to an optical axis of the gain medium. This helps to prevents reflections from the cavity facing (rear) facet from being coupled back into the cavity, improving the likelihood that the cascade laser will operate on modes of the combined cavity and improving stability of the cascade laser. The oblique angle may be between 2 degrees and 16 degrees.

The mid-infrared cascade laser may further comprise an additional optical element in the cavity to increase the effective length of the cavity. The additional optical element is transparent at midinfrared wavelengths and may have a refractive index > 2, such as germanium. This additional optical element may increase the effective cavity length of the cavity, for multi-mode operation.

The cascade gain medium may have a facet coated with a reflective coating to act as a high reflector. The cavity may comprise a cavity mirror coated with a partially reflective coating to act as an output coupler. The cavity may further comprise a lens configured to collimate laser emission from the cascade gain medium which is transmitted through the cavity mirror. This arrangement has a benefit of using the cavity lens to also provide collimated laser output, removing the need for additional optics to collimate the laser emission.

According to a second aspect of the invention, there is provided a method of tuning a mid-infrared cascade laser. The method comprises generating a set of longitudinal resonator modes from a cascade gain medium and a cavity, the set comprising a plurality of longitudinal resonator modes of the cavity. The method also comprises aligning a first optical filtering element in the cavity to select a first subset from the set of longitudinal resonator modes. The method also comprises aligning a second optical filtering element in the cavity to select a second subset of longitudinal resonator modes from the first subset.

The first optical filtering element may be a bandpass filter, such as a bandpass interference filter. The second optical filtering element may be an etalon. The second optical filtering element may select a single longitudinal resonator mode. For example, a single longitudinal resonator mode required for a particular application, such as a single longitudinal resonator mode that overlaps with a gas absorption line.

Aligning the first and/or second optical filtering elements may involve adjusting the angle of the first and/or second optical filtering elements.

-6The method may further comprise adjusting an effective length of the cavity to adjust the wavelength of the second subset, for example, to fine tune the cascade laser to a particular wavelength of a gas absorption line. The effective length of the cavity may be adjusted by moving a cavity mirror or adjusting the temperature of the cascade laser.

According to a third aspect of the invention, there is provided a method of gas detection using a mid-infrared cascade laser. The method of gas detection may comprise: generating a set of longitudinal resonator modes from a cascade gain medium and a cavity, the set comprising a plurality of longitudinal resonator modes of the cavity, wherein the set of longitudinal resonator modes includes a wavelength of an absorption line of a gas to be detected; aligning a first optical filtering element in the cavity to select a first subset from the set of longitudinal resonator modes, wherein the first subset includes the wavelength of the absorption line; aligning a second optical filtering element in the cavity to select a second subset of longitudinal resonator modes from the first subset; and adjusting the second optical filtering element to tune the second subset of longitudinal resonator modes to overlap with the wavelength of the absorption line. This allows for a cascade gain medium, with an emission spectrum which is close to a gas absorption line but not quite ideal (for example, slightly shifted in wavelength) to be adjusted until the emission wavelength better overlaps the gas absorption line. In this way, less fabricated cascade lasers with less than ideal emission wavelengths are wasted, reducing the cost to manufacture cascade lasers and therefore the price to purchase a cascade laser. This provides an effective way to tune a cascade laser for gas detection which is cheaper and easier than DFB, and more compact and reliable than using an external cavity diffraction grating.

The method of gas detection may further comprising adjusting an effective length of the cavity to adjust the wavelength of the second subset, for example, to fine tune the cascade laser to a particular wavelength of a gas absorption line.

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 cross-section through a cascade laser according to an embodiment of the invention;

-7 Figure 2a illustrates potential longitudinal modes of the cascade laser of Figure 1;

Figure 2b illustrates the transmission bandwidth of an interference filter in the cascade laser of Figure 1;

Figure 2c illustrates a first subset of longitudinal modes selected by the interference filter; Figure 2d illustrates the transmission spectrum of an etalon in the cascade laser of Figure 1; Figure 2e illustrates a second subset of longitudinal modes selected by the interference filter and the etalon in combination;

Figure 3 illustrates a cross-section through a cascade laser according to an alternative embodiment of the invention;

Figure 4a illustrates an experimental emission spectrum for the cascade gain medium in the cascade laser of Figure 1;

Figure 4b illustrates an experimental emission spectrum of the cascade laser of Figure 1 with an interference filter in the external cavity; and

Figure 4c illustrates an experimental emission spectra of the cascade laser of Figure 1 with an interference filter and an etalon in the external cavity for four different etalon angles (7.8°, 8.7°, 9.5° and 10.4°) showing how the single mode emission peak may be tuned by changing the etalon angle.

Detailed Description

Figure 1 illustrates a mid-infrared laser 100, which operates in a single longitudinal mode, or a small number of longitudinal modes, to provide a narrow wavelength range somewhere in the midinfrared region (2 pm to 20 pm) which can be targeted to a particular application, such as a specific gas absorption line for gas sensing.

The mid-infrared laser 100 has a cascade gain medium 110 with a broad gain spectrum, such as a quantum cascade gain medium or an interband cascade gain medium. The gain medium 110 has a front facet 111 and a rear facet 112. An external cavity 130 is formed between the rear facet 112 and a cavity mirror 132. The gain medium 110 emits a laser beam 114 from the rear facet 112 which is collimated by lens 134. Lens 134 also couples the laser beam 114 from the external cavity 130 back into the gain medium 110.

Reflections from the rear facet 112 give rise to the possibility that the cascade laser 100 will operate on longitudinal modes (Figure 2a) of one of three separate cavities: the external cavity 130, the gain

-8medium cavity 134, or a combined laser cavity 136 incorporating the external cavity 130 and the gain medium cavity 134. Operating on modes of either the gain medium cavity 134 or the external cavity 130 is undesirable, as the gain medium cavity 134 and external cavity 130 tend to compete for dominance causing instability and mode hopping. It is preferred for the cascade laser 100 to operate on modes of the combined laser cavity 136 instead. To encourage the cascade laser 100 to operate on the preferred combined laser cavity 136, the rear facet 112 is anti-reflection coated to eliminate reflections from the rear facet 112 as much as possible. Minimising reflections from the rear facet 112 also improves feedback from the external cavity 130 back into the gain medium 110. Preferably, reflections from the rear facet 112 are 0.1% or less, more preferably 0.01% or less.

An interference filter 140 is positioned between the lens 134 and the cavity mirror 132. Relying on the interference filter 140 alone (as in Kischkat) to select a single longitudinal mode has problems, since narrow transmission bandwidth interference filters suitable to select a single mode tend to have low transmission efficiency (typically around 50% or less). Such low transmission efficiency will reduce feedback from the external cavity 130 back into the gain medium 110, reducing the suppression of rejected wavelengths and resulting either in unstable single mode operation or more likely multi-mode operation.

The suppression of rejected wavelengths improves as the transmission efficiency of the interference filter 140 is increased, and mid-infrared interference filters with a much higher transmission efficiency (85% or more) can be easily produced. However, high transmission efficiency interference filters in the mid-infrared tend to only have much broader transmission bandwidths (of around 0.5 1% of the peak transmission wavelength of the interference filter 140) which is not sufficient to select a single longitudinal mode. So, instead the interference filter 140 has a transmission bandwidth (Figure 2b) which selects a first subset of the plurality of emitted longitudinal modes (Figure 2c), supressing laser emission at the wavelengths that are rejected by the interference filter 140.

As the high transmission bandwidth interference filter 140 used in the cascade laser 100 cannot select a single mode by itself, the interference filter 140 is combined with an etalon 142 which selects a narrower second subset of the plurality of longitudinal modes selected by the interference filter 140.

-9The etalon 142 may be produced from a highly polished and parallel plate, for example, with a tilt error of less than around 2 arcsec. The plate for the etalon 142 is made from a material transparent in the mid-infrared range, for example, fused silica, sapphire, calcium fluoride, silicon, germanium or chalcogenide glasses. The plate is then cut into pieces with the size required for a compact laser package. The required finesse of the etalon 142 is achieved by partly reflection coating (providing more than around 70% reflection) on both sides of the etalon 142. Higher finesse values are required for narrower wavelength band selection. The effective thickness of the etalon 142, taking into account the refractive index of the material, is chosen to provide the required free spectral range. For single mode operation, the free spectral range of the high finesse etalon 142, that is, the energy separation between its Fabry-Perot fringes Av_et (Figure 2d), needs to be larger than the emission bandwidth Avl (Figure 2c) of the cascade laser 100 operating with the interference filter 140 of bandwidth Avf (Figure 2b).

To align the cascade laser 100, the angle of the interference filter 140 is adjusted to select the first subset of longitudinal modes from the plurality of potential longitudinal modes of the laser cavity. Ideally, the desired emission wavelength of the cascade laser 100 is located at or close to a point of maximum transmission efficiency of the interference filter 140 to enhance the efficiency of the cascade laser 100.

An angle of the etalon 142 is then adjusted to select a second subset of longitudinal modes from the first subset. Ideally, the etalon 142 will be used to select a single longitudinal mode (Figure 2e) which is as close as possible to a wavelength required for a particular application, for example, in gas sensing the longitudinal mode will be selected which corresponds most closely with a particular gas absorption line for trace gas spectroscopy, detection and/or monitoring.

All of the components of the mid-infrared laser 100 may be housed in a compact package 150, such as a butterfly package. Once the desired emission wavelength has been achieved, the interference filter 140 and the etalon 142 may be fixed in position, for example, by fixing them in place in the compact package 150 with an optical glue or potting compound.

The gain medium 110 is fixed to a submount 190 (for example, by soldering) which may act as a heat spreader. This may be particularly important with cascade lasers based on a quantum cascade gain medium which may be less efficient than those using an interband cascade gain medium.

-10The submount 190, lens 134, interference filter 140, etalon 142 and cavity mirror 132 are attached to a mount 180, which may be temperature controlled by a thermal electric cooler. To reduce temperature induced movement of the components, and thereby minimise fluctuations in the energy positions of the longitudinal modes, the submount 190, the mount 180 and/or the thermoelectric cooler may be made from materials with high thermal conductivity but low thermal expansion coefficients (for example, ceramic, AIN, diamond, or CuW alloys).

Further fine tuning of the single mode emission (for example, to finely change the emission wavelength during a measurement, or for precise alignment with a particular gas absorption line) may be achieved by, for example, adjusting the temperature of the cascade laser 100, because varying the temperature of the cascade laser 100 influences the refractive indices of the optical elements of the cascade laser 100, causing the spacing of the longitudinal modes to change. Additionally or alternatively, the length of the combined cavity 136 may be changed by adjusting the position of the cavity mirror 132 using a positioning mechanism 160. The positioning mechanism 160 may be a piezoelectric transducer to which the cavity mirror 132 may be mounted or glued.

The output 125 from the front facet 111 exits the package 150 through a hole in its wall covered with an optional output window 157 made from a material transparent in the mid-infrared range. Transmission of the window 157 may be maximised by anti-reflection coating the window 157 or by choosing a material for the window 157 with a low refractive index.

It can be difficult to resolve a particular single mode if the longitudinal modes of the cascade laser 100 are closely spaced. This is a particular problem when the cascade laser 100 is operating in a pulsed mode where strong temperature-induced shifts of the longitudinal modes may occur during a pulse. The energy difference between neighbouring longitudinal modes is inversely proportional to the length of the operating laser cavity. The location of the interference filter 140 and etalon 142 inside the external cavity 130 can be chosen arbitrarily, but to maximise cavity mode separation energy, the length of the operating laser cavity should be minimised. By creating a laser cavity which is as short as possible, the longitudinal modes will be positioned further away from each other and the likelihood of mode hopping will be reduced. In the example of a gain medium 110 formed from a 2 mm long cascade laser diode with a separation distances of, for example, 0.3 mm between each optical component (which are each about 0.5 mm -1 mm thick), a maximum achievable longitudinal mode separation is less than 0.4 cm¹ for a combined cavity 136, which is a suitable wavelength tuning range for trace gas detection/spectroscopy.

-lilt can be challenging to reduce reflections from the rear facet 112 to 0.01 % or less given the highly divergent nature of the laser beam 114 emitted by the gain medium 110 and the resulting increased reflectivity for emission angles which are highly divergent from the normal of the surface of the rear facet 112. Figure 3 illustrates a cascade laser 200 which seeks to solve this problem. The gain medium 210 has been fabricated so that its rear facet 212 is at an angle to the optical axis of the gain medium to reduce retro-reflections that could be coupled back into the laser cavity. The rear facet 212 is angled by, for example, forming a laser ridge which is curved towards one end before cleaving or by etching the rear facet 212 at an angle between 2 degrees and 16 degrees. The combination of an angled rear facet 212 with an anti-reflection coating with low residual reflectivity reduces reflections from the rear facet 212 that are coupled back into the laser cavity to 0.01 % or less, improving the reliability with which the cascade laser 200 operates preferentially on the combined cavity 136.

So that the laser emission from the angled rear facet 212 is directed towards the cavity mirror 132, the gain medium 210 is soldered onto submount 190 at an angle relative to the optical axis of the external cavity 130. With the gain medium 210 at an angle, laser emission from the front facet 211 would also be angled with respect to the optical axis of the external cavity 130, which makes it difficult to provide laser emission from the front facet 211 particularly when the laser 200 is housed in a compact package 150 where there is little room for additional beam steering optics. To solve this problem, the front facet 211 may be coated in a highly reflective coating so that the front facet 211 acts as the high reflector. In this case, the cavity mirror 232 is partially reflective to act as the output coupler. This arrangement has a benefit of providing collimated laser emission 225, removing the need for additional collimating optics that may be required for the cascade laser 100 in Figure 1 where the laser emission 125 is highly divergent.

For some applications, such as mid-infrared spectroscopy and detection of complex gas molecules where the absorption spectrum has very densely positioned lines, the use of a single mode laser operating on a precisely defined wavelength may be unnecessary. Instead, it may be possible to use a laser operating with a particular narrow emission spectrum (< 1 - 2 cm¹) without necessarily having to be single mode, which has an advantage of providing the required species selectivity while covering several absorption lines simultaneously for increased detection sensitivity. To achieve narrow, but not necessarily single mode, operation the separation between longitudinal modes is reduced by increasing the length of the combined cavity 136. The length of the combined cavity 136

-12may be increased by using a longer gain medium 210 and/or by increasing the distance between the gain medium 210 and the cavity mirror 232.

However, to maximise the length of the combined cavity 136 in a compact package, there may be insufficient room to increase the distance between the gain medium 210 and cavity mirror 232, or to increase the length of the gain medium 210. In this case, it may be advantageous to introduce an additional antireflection coated optical element 270 with a high refractive index (such as germanium or silicon) into the external cavity 130, to increase the effective cavity length. This additional optical element 270 could be combined into one element with the cavity mirror 132 and/or the interference filter 140 by depositing the cavity mirror 132 on the back surface of the additional optical element and, optionally depositing the interference filter 140 on the front surface.

A standard butterfly package has an internal length of around 17 mm. Taking into account the refractive indices of the optical elements, an effective laser cavity of around 55 mm, can be formed inside such a butterfly package. With a 3 mm long cascade laser chip and a 10 mm long additional optical element made from germanium, the mode separation is less than 0.1 cm¹. As a result, the emission spectrum of the laser will be multi-mode with more than 10 emitting modes for a 1 - 2 cm¹transmission band of etalon 142. Fine tuning of the narrow multimode emission is provided by modulation of the incident angle of the etalon. The etalon in this case is mounted on, for example, a piezo actuator which provides angular/rotational movement.

Example

Figure 4 illustrates experimentally measured room temperature spectra for the cascade laser 100 of Figure 1, measured using a Fourier transform infrared (FTIR) spectrometer. In this example, a quantum cascade gain medium 110 is 3 mm long and has a gain spectrum centred around 5.3 pm wavelength (1886 cm¹). The rear facet 112 is not angled but is anti-reflection coated with a dielectric coating, such that the reflectance of the rear facet 112 is around 0.1 %. The front facet 111 is not anti-reflection coated and is as-cleaved.

The position of the cavity mirror 132 is arranged such that the mirror is 5.7 mm away from the rear facet 112. The lens 144 is a 0.85 numerical aperture chalcogenide aspheric lens, which is 0.9 mm thick and has a refractive index of 2.65. The interference filter 140 is deposited on a 1 mm thick germanium plate and has a transmission bandwidth of around 2 % relative to the peak transmission

-13wavelength of the interference filter 140 of 1897 cm¹. The peak transmission efficiency of the interference filter 140 is about 88 %. The etalon 142 is 0.146 mm thick and has a free spectral range of about 10 cm¹. The etalon 142 is made from silicon which has been coated on the front and rear surfaces to give approximately 70 % reflectivity. The quantum cascade laser has a laser threshold of approximately 200 mA, driven by 50 ns long pulses. The emission spectra in Figure 4 have been obtained at a drive current of 300mA.

Figure 4a shows the emission spectrum without an interference filter 140 or etalon 142 in the external cavity 130, such that the quantum cascade laser 100 operates on a plurality of longitudinal modes and emits a broad wavelength spectrum, with a mode separation energy of around 0.7 cm¹. Taking into account the refractive index of the lens 134, such a mode separation energy can only correspond to the effective length of the external cavity 130 (approximately 7.1 mm). Therefore, for these conditions, due to a relatively high reflectivity of the rear facet 121, the quantum cascade laser operates on the modes of the external cavity 130 and not on the modes of the combined cavity 136. This emission spectrum was measured with a resolution of 0.125 cm¹

As shown in Figure 4b, the emission spectrum of the cascade laser 100 becomes much narrower if an interference filter 140 is inserted inside the external cavity 130 at an incident angle close to the normal angle. However, the emission spectrum in Figure 4b is still exhibiting several modes. This spectrum has been measured with the resolution of 0.25 cm¹.

After adding a high finesse etalon 142 into the external cavity 130 in addition to the interference filter 140, the emission spectrum shows only a single longitudinal mode. By changing the incident angle of the etalon 142, any required single mode emission can be selected between neighbouring longitudinal modes in a 10 cm¹ wide wavelength range, which is only limited by the free spectral range of the etalon 142. Figure 4c shows examples of four different single mode laser emission spectra corresponding to four different angles of the etalon 142 (7.8°, 8.7°, 9.5° and 10.4°). These spectra have been measured with the resolution of 0.125 cm¹ and are plotted on a vertically shifted scale for clarity. The measured width of the single mode emission peak is 0.14 cm¹ (limited by the spectrometer resolution).

Surprisingly, after the interference filter 140 and the etalon 142 have been introduced into the external cavity 130, the quantum cascade laser starts to operate on the modes of the laser gain medium 110 (cavity 134), where mode separation energy is close to 0.5 cm¹. Potentially unstable

-14behaviour may arise from operating on the modes of the gain medium cavity 134, such as unstable switching between the modes of the external cavity 130 and the gain medium cavity 134. This unstable behaviour may be eliminated by using the angled rear facet geometry discussed in relation to Figure 3 to encourage the cascade laser to operate on the modes of the more stable combined cavity 136.

Although the invention has been described in terms of certain preferred embodiments, the skilled person will appreciate that various modifications can be made which still fall within the scope of the appended claims.

The lens 134 may be antireflection coated. The lens 134 may be a ball lens (which is economical but less efficient), or a high numerical aperture 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 is made from a material that is transparent in the mid-infrared range, for example, fused silica, sapphire, calcium fluoride, silicon, germanium or chalcogenide glasses.

The interference filter 140 and the cavity mirror 132 may be formed as a single optical element. For example, the interference filter 140 may be deposited on the cavity mirror 132. Combining the interference filter 140 and the cavity mirror 132 reduces the number of optical components required to build the cascade laser 100, making the cascade laser 100 cheaper to manufacture and easier to align because there are less degrees of freedom, which is a particular advantage for building compact laser packages.

The cascade laser 100 may include a spectrometer or other wavelength measuring equipment to measure the emission spectrum of the cascade laser 100. The measured emission spectrum may be used to control a feedback loop which controls a tuneable optical element (such as the cavity mirror 132) or the temperature of the cascade laser 100 in order to maintain a desired emission spectrum of the cascade laser 100.

The interference filter 140 may be one of: a narrow band pass filter; and a longpass edge filter in combination with a shortpass edge filter.

-15To improve single mode selectivity, if necessary, more than one high finesse etalon 142 may be inserted into the external cavity 130, or the finesse of the etalon 142 may be increased by increasing the reflectivity of the coatings on the surfaces of the etalon 142.

Instead of attaching the submount 190, lens 134, interference filter 140, etalon 142 and cavity mirror 132 to a mount 180, which is cooled by a thermoelectric cooler, the submount 190, lens 134, interference filter 140, etalon 142 and cavity mirror 132 may be attached directly to the thermoelectric cooler.

The interference filter 140 may be tuneable. For example, the interference filter 140 may have an interference pattern that varies across the interference filter (such as a variable linear filter) where the wavelengths transmitted by the interference filter are controlled by moving the filter relative to the laser beam. Additionally, or alternatively, the angle of the interference filter may be adjusted to tune the wavelengths transmitted by the interference filter.

Claims

1. A mid-infrared cascade laser comprising:

a cascade gain medium and a cavity configured to generate a set of longitudinal resonator modes, the set comprising a plurality of longitudinal resonator modes of the cavity;

the cavity comprising a first optical filtering element and a second optical filtering element, wherein the first optical filtering element selects a first subset from the set of longitudinal resonator modes and the second optical filtering element selects a second subset of longitudinal resonator modes from the first subset.

2. The mid-infrared cascade laser of claim 1, wherein:

the set of longitudinal resonator modes contains more longitudinal modes than the first subset of longitudinal resonator modes; and the first subset of longitudinal resonator modes contains more longitudinal resonator modes than the second subset of longitudinal resonator modes.

3. The mid-infrared cascade laser of either preceding claim, wherein the second subset of longitudinal resonator modes comprises a single longitudinal resonator mode.

4. The mid-infrared cascade laser of any preceding claim, wherein the first optical filtering element is a bandpass filter and the second optical filtering element is an etalon.

5. The mid-infrared cascade laser of claim 4, wherein the bandpass filter has a transmission efficiency of one of: greater than 50 %; greater than 60 %; greater than 70 %; greater than 75 %; greater than 80 %; greater than 85 %; greater than 90%; and greater than 95%.

6. The mid-infrared cascade laser of either of claims 4 or 5, wherein the bandpass filter is tuneable.

7. The mid-infrared cascade laser of claim 6, wherein the tuneable bandpass filter comprises one of: an angle tuneable bandpass interference filter and a variable linear filter.

8. The mid-infrared cascade laser of any of claims 4 to 7, wherein the etalon has an effective thickness such that the free spectral range of the etalon is wider than the first subset of longitudinal modes.

9. The mid-infrared cascade laser of any preceding claim wherein the transmission bandwidth of the bandpass filter is one of: greater than 0.5%; and greater than 1%.

10. The mid-infrared cascade laser of any preceding claim, wherein the mid-infrared wavelength emitted by the mid-infrared cascade laser is in the range of 2 pm and 20 pm.

11. The mid-infrared cascade laser of any preceding claim, wherein the cascade gain medium comprises a quantum cascade gain medium or an interband cascade gain medium.

12. The mid-infrared cascade laser of any preceding claim, further comprising a device configured to change the effective length of the cavity.

13. The mid-infrared cascade laser of claim 12, wherein the device is one of: an actuator configured to move a cavity mirror defining the cavity; and a temperature controller configured to adjust a temperature of the cascade laser to change the effective length of the cavity.

14. The mid-infrared cascade laser of any preceding claim, wherein the cascade gain medium has a cavity facing facet that is coated with an anti-reflection coating having a residual reflectivity of less than 0.1 %, or less than 0.01%.

15. The mid-infrared cascade laser of claim 14, wherein the cavity facing facet is at an oblique angle with respect to an optical axis of the gain medium.

16. The mid-infrared cascade laser of any preceding claim, further comprising an additional optical element in the cavity to increase the effective length of the cavity.

17. The mid-infrared cascade laser of any preceding claim, wherein the cascade gain medium has a facet coated with a reflective coating to act as a high reflector and the cavity comprises a cavity mirror coated with a partially reflective coating to act as an output coupler, wherein the cavity further comprises a lens configured to collimate laser emission from the cascade gain medium which is transmitted through the cavity mirror.

18. A method of tuning a mid-infrared cascade laser comprising:

generating a set of longitudinal resonator modes from a cascade gain medium and a cavity, the set comprising a plurality of longitudinal resonator modes of the cavity;

aligning a first optical filtering element in the cavity to select a first subset from the set of longitudinal resonator modes; and aligning a second optical filtering element in the cavity to select a second subset of longitudinal resonator modes from the first subset.

19. The method of claim 18, wherein the first optical filtering element is a bandpass filter and the second optical filtering element is an etalon.

20. The method of either of claims 18 or 19, wherein the second optical filtering element selects a single longitudinal resonator mode.

21. The method of any of claims 18 to 20, further comprising adjusting an effective length of the cavity to adjust the wavelength of the second subset.

22. The method of claim 21, wherein the effective length of the cavity is adjusted by moving a cavity mirror or adjusting the temperature of the cascade laser.

23. A method of gas detection using a mid-infrared cascade laser comprising:

generating a set of longitudinal resonator modes from a cascade gain medium and a cavity, the set comprising a plurality of longitudinal resonator modes of the cavity, wherein the set of longitudinal resonator modes includes a wavelength of an absorption line of a gas to be detected;

aligning a first optical filtering element in the cavity to select a first subset from the set of longitudinal resonator modes, wherein the first subset includes the wavelength of the absorption line;

aligning a second optical filtering element in the cavity to select a second subset of longitudinal resonator modes from the first subset; and adjusting the second optical filtering element to tune the second subset of longitudinal resonator modes to overlap with the wavelength of the absorption line.

24. The method of gas detection of claim 23, further comprising adjusting an effective length of the cavity to adjust the wavelength of the second subset.