"Narrow linewidth and frequency tunable cat-eye external cavity diode lasers"

Technical Field

The present application describes a narrow linewidth and frequency tunable cat-eye external cavity diode laser .

Background art

Within different branches of emerging technologies there is an increasing demand and use for laser technology and application with narrowband wavelengths and tunable emission frequencies . One of the related industries is the automotive with the Frequency Modulation in Continuous Wave Light Detection and Ranging (LiDARs ) .

As a consequence of the coherence length of the used lasers in this industry being required to be larger than double of the range of the sensor , sub MHz linewidth frequency tunable light sources are therefore imposed . These frequency tunable light sources are also required to be lightweight and small in size , as well as robust against vibrations and thermal drifts .

The tunable lasers have also increased potential in more canonical time-of-f light ( ToF) LiDARs , as the possibility to tune out of the frequency, or equivalently wavelength, of another LiDAR operating in the surrounding is beneficial to avoid LiDAR cross-interferences . Though, in this case linewidth constraints are more relaxed, as there is no need for long coherence lengths . Another automotive application, for which narrow linewidth tunable lasers are beneficial is the Road Condition Sensing, as this allows to perform spectroscopy measurements on water and ice . For this application power and frequency stability is also of crucial importance , depending on the specific implementation .

The more commonly used laser light sources nowadays are :

• SIMPLE LASER DIODES - Large linewidths and low cost .

• EXTERNAL CAVITY DIODE LASERS ( ECDL ) - In general they provide narrow-band / ultra-narrow-band linewidths and tunability up to tens of nanometers . In general , require large masses for thermal stability, long cavity lengths , resulting in large devices ( up to shoebox sized devices and weighting a few kilograms ) . Proposed in three configurations , Littrow, Littman-Metcalf and Cat eye : o Littrow and Littman-Metcalf configuration :

■ Require high mechanical stability;

■ Require high thermal stability;

■ Non self-aligning (wavelength tuning produces beam steering ) . o Cat eye configuration :

■ Two external cavities nested into each other, the inner one being an etalon;

■ Self-aligning ;

■ Reduced size with regard to Littrow configurations ;

■ About 20 nanometers coarse tuning;

■ About 10-15 GHz mode-hop free frequency scanning ;

■ Improved acoustic and mechanical stability . • DISTRIBUTED BRAGG REFLECTOR ( DBR) LASERS - Mechanically stable , with tunability of 40 GHz in mode hop free , low power diodes cost in between few hundreds up to above a thousand dollars , typical bandwidth in the order of a few 100 kHz up to about 4 MHz .

• TUNEABLE VOLUME -HOLOGRAPHIC-GRATING (VHG) / VOLUME BRAGG GRATING (VBG ) LASERS - Reasonable high power ( in the 10 mW to 500 mW range ) , linewidth of about 100 kHz are usually achieved, tunability can be achieved with forward current up to tens of GHz and relatively low cost .

• TUNEABLE VERTICAL-CAVITY SURFACE-EMITTING LASER (VCSEL ) - Cheaper than Distributed feedback laser ( DFB ) / DBR lasers and tuneable over several nm, but linewidths are seldom smaller than 20 MHz .

Although the wide range of existing technologies , not all of the existing architectures are suitable for highly portable and narrow linewidth devices . Some architectures , e . g . , the ECDL in Littrow or Littman-Metcalf configurations or the more compact cat-eye configuration, provide tunable lasers at narrow / ultra-narrow linewidths but are bulky and heavy and usually expensive . Others are cheaper and smaller but present linewidths do not narrow enough for application in Frequency-Modulated Continuous Wave LiDAR ( FMCW ) as the coherence length is too short for ~100m range LiDARS , or atomic physics applications which require linewidths comparable or lower than 1 to 2 MHz , depending on the specific application, e . g . , vertical-cavity surface-emitting laser (VCSEL ) . Therefore, the present invention aims to solve all the above- mentioned limitations of the state-of-the-art technologies .

Summary

The present invention describes an external cavity diode laser comprising a laser diode emitting source configured to emit a light beam within an optical path; a first deflector collimating lens , positioned within the optical path of the light beam; a second deflector collimating lens , positioned within the optical path of the light beam; a tunable filter, positioned within the optical path of the light beam between the first deflector collimating lens and the second deflector collimating lens ; an output coupling mirror and an output collimating lens sequentially positioned in the optical path of the light beam after the second deflector collimating lens ; wherein an emitted light beam from the laser diode emitting source is tunable through tilting different angles with regard to the optical path of said emitted light beam, resulting in an output light beam with an adj usted wavelength .

In a proposed embodiment of present invention, the first deflector collimating lens comprises a set of a collimating lens and a first deflector A .

Yet in another proposed embodiment of present invention, the second deflector collimating lens comprises a set of a second deflector B and a cat-eye lens .

Yet in another proposed embodiment of present invention, the laser diode emitting source comprises one of an Fabry-Perot laser diodes such as edge-emitting diode lasers or verticalcavity surface-emitting laser (VCSEL ) .

Yet in another proposed embodiment of present invention, the collimating lens comprises one of a metalens or a Fresnel lens or other microfabricated lens , as well as normal refractive lens .

Yet in another proposed embodiment of present invention, the collimating lens are configured to collimate the laser light produced by the laser diode .

Yet in another proposed embodiment of present invention, the first deflector A comprises at least one of a normal prism or preferably a grating optimized for diffraction in one specific order , usually the first order .

Yet in another proposed embodiment of present invention, the first deflector A ensures that a light reflected by the tunable filter does not couple back into the laser diode .

Yet in another proposed embodiment of present invention, the tunable filter comprises one of a MicroElectroMechanical mechanism configured to adj ust and tune the emitted light beam .

Yet in another proposed embodiment of present invention, the second deflector B is structurally equal to the first deflector A and is configured to provide the opposite effect of the first deflector A. Yet in another proposed embodiment of present invention, the cat-eye lens comprises one of a collimating lens and its configured to focus the light beam.

Yet in another proposed embodiment of present invention, the output coupling mirror comprises one of a tunable filter operating as a tunable semi-reflective mirror micro actuated through a MicroElectroMechanical mechanism, compensating the effect of the first deflector A, and ensuring that an external light source does not couple back into the laser diode .

General Description

The present application describes a narrow linewidth and frequency tunable cat-eye external cavity diode laser incorporating MicroElectroMechanical Systems (MEMS ) configured to tune the two existing external nested cavities of the cat eye laser .

The proposed technology disclosure is related with the increasing needs of quantum technology, were narrow linewidth tunable lasers are widely used for various tas ks , such as : laser cooling , trapping of neutral alkali atoms , hyperfine levels spectroscopy, absorption imaging, Bose- Einstein condensation, trapped ion quantum computing and time frequency standards among others .

Depending on the specific application, the following requirements are desirable to be achieved :

• Power stability; • Robust against vibrations (mainly for non-laboratory applications ) ;

• Robust against temperature drifts ;

• Large mode-hopping free tunability range ;

• Polarization stability ( depending on application, e . g . Magneto Optical Traps ) ;

• Narrow / ultra-narrow (< 100 kHz ) linewidths

• Single frequency;

When compared with the previously mentioned laser technologies , this sensor layout arrangement provides the advantages of the cat-eye laser configuration with the added value of a reduced size and weight , as well as the reduction of optical components . As the filtering is not achieved through piezoelectric driven mechanical rotation of an etalon but instead with a MEMS device , the overall size of this laser configuration can be significantly reduced . Furthermore , with the use of diffractive lenses within this arrangement allows the used optics to be reduced in size without compromising the narrow linewidth specs of the laser . In the usual cat-eye laser the external cavity is composed by two 'nested' cavities , the long one and the inner one . Since the inner one is an etalon, being its medium is a solid dielectric material , not being possible to adj ust the distance in between the two mirrors to select the wavelength, it will be tunned through tilting different angles with regard to the optical axis .

The configuration of the proposed invention allows to obtain compact designs in the sub cm scale and tens of grams in weight , and enhance cheaper production thanks to the use of MEMS technology components , with high wavelength tunability, in the range of few GHz , and ultra-narrow bandwidth, potentially in the lOKHz range .

Brief description of the drawings

For better understanding of the present application, figures representing preferred embodiments are herein attached which, however, are not intended to limit the technique disclosed herein .

Fig . 1 - illustrates a possible embodiment of the proposed narrow linewidth and frequency tunable cat-eye external cavity diode laser , in a cross-section perspective , wherein the reference numbers relate to :

100 - narrow linewidth and frequency tunable cat-eye external cavity diode laser;

1 - laser diode emitting source ;

2 - collimating lens ;

3 - first deflector A;

4 - tunable Fabry-Perot filter;

5 - second deflector B;

6 - cat-eye lens ;

7 - output coupling mirror ;

8 - output collimating lens ;

20 - emitted light beam;

21 - output light beam.

Fig . 2 - illustrates another possible embodiment of the proposed narrow linewidth and frequency tunable cat-eye external cavity diode laser , in a cross-section perspective , wherein the reference numbers relate to : 100 - narrow linewidth and frequency tunable cat-eye external cavity diode laser;

1 - laser diode emitting source ;

4 - tunable Fabry-Perot filter;

7 - output coupling mirror ;

8 - output collimating lens ;

9 - first deflector collimating lens ;

10 - second deflector collimating lens ;

20 - emitted light beam;

21 - output light beam.

Fig . 3 - illustrates another possible embodiment of the proposed narrow linewidth and frequency tunable cat-eye external cavity diode laser , in a cross-section perspective , wherein the reference numbers relate to :

100 - narrow linewidth and frequency tunable cat-eye external cavity diode laser;

1 - laser diode emitting source ;

2 - collimating lens ;

11 - Pin Hole ( PH ) ;

12 - Hyper Chromatic Lens ( HCL ) ;

20 - emitted light beam;

21 - output light beam.

Fig . 4 - illustrates the transmission spectra of the narrow linewidth and frequency tunable cat-eye external cavity diode laser ( 100 ) , wherein the reference numbers relate to :

200 - cumulative effect transmission spectra;

201 - laser diode cavity transmission spectra ;

202 - external cavity transmission spectra ;

203 - laser medium gain transmission spectra;

204 - total tuned spectrum of the Fabry-Perot filter ;

205 - spectrum of the tunable Fabry-Perot filter . Description of Embodiments

With reference to the figures, some embodiments are now described in more detail, which are however not intended to limit the scope of the present application.

The structure of the device (100) herein disclosed, the narrow linewidth and frequency tunable cat-eye external cavity diode laser, can comprise several embodiments.

In one of the possible configurations, the cat-eye ECDL (100) comprises a laser diode emitting source (1) , a collimating lens (2) , a first deflector A (3) , a tunable Fabry-Perot filter (4) , a second deflector B (5) , a cat-eye lens (6) , an output coupling mirror (7) and an output collimating lens (8) .

The laser diode emitting source (1) comprises one of an Fabry-Perot laser diodes, such as edge-emitting diode lasers or vertical-cavity surface-emitting laser (VCSEL) . The collimating lens (2) comprises one of a metalens (binary diffractive lens, that achieves effective index of refraction modulations with sub wavelength structures) or a Fresnel lens or other microfabricated lens, as well as normal refractive lens; are configured to collimating the laser light produced by the laser diode (1) . The first deflector A (3) comprises at least one of a normal prism or preferably a grating optimized for diffraction in one specific order, usually the first order. This element (3) ensures that the light rejected, i.e., reflected, by the tunable filter (4) , does not couple back into the laser diode (1) . The tunable Fabry-Perot filter (4)... The second deflector B (5) is structurally equal to the first deflector A (3) , but it is arranged in order to provide the opposite effect when compared with said deflector A (3) . The cat-eye lens (6) , in one of the preferred embodiments uses the same technology as the one used in the collimating lens (2) , and it is configured to focus the collimated light beam (20) . The output coupling mirror (7) , uses an equivalent technology and is axially arranged with the tunable Fabry- Perot filter (4) , working as a tunable actuated semi- reflective mirror (ASRM) micro actuated through a MEMS mechanism. The mirror (7) compensates the effect of the first deflector A (3) , as well as it enables external light from coupling into the laser diode (1) . The output collimating lens (8) yields a collimated beam (21) in output from the device ( 100 ) .

In another of the possible embodiment of the proposed the cat-eye ECDL (100) the collimating lens (2) and the first deflector A (3) are comprised in a single element (9) through microfabrication techniques adding the phase mask of the two optical elements. Additionally, in this proposed embodiment, the second deflector B (5) and the cat-eye lens (6) are also merged together in a single element (10) . Therefore, in this second possible embodiment, the cat-eye ECDL (100) comprises a laser diode emitting source (1) , a first deflector collimating lens (9) , a tunable Fabry-Perot filter (4) , a second deflector collimating lens (10) , an output coupling mirror (7) and an output collimating lens (8) .

Finally, in the third possible embodiment of present invention, the cat-eye ECDL (100) comprises a collimating lens (2) , a first HCL (12) , a PH (11) and a second HCL (12) . The proposed device (100) arrangement, i.e. , the MEMs Cat-eye External Cavity Diode Laser, comprises three optical cavities.

The first optical cavity is the cavity of the laser diode (1) itself. The second optical cavity is represented by the tunable filter (4) . The third optical cavity is represented by the cavity formed by the output surface of the diode laser (1) and the outcoupling mirror (7) .

While the laser diode (1) cavity is fixed, and might be changed only through thermal expansion, the remaining two, i.e. , the tunable filter (4) and the outcoupling mirror (7) , are tunable and can be used to select the emission mode of the overall device (100) . With a combined action of these last two tunable cavities (4, 7) , actuated through MEMS technology, it is possible to scan the output frequency over a few GHz and, depending on the reflectivity of the various mirrors involved and separation in between those, achieve ultra-narrow linewidths.

In the herein disclosed embodiment of the invention, the lenses (2, 6, 8) comprise very thin diffractive lenses, built through cheap and mass production scale lithographic methods. These thin lenses allow to reduce mass and through lithographic techniques reduce costs. . The proposed particular arrangement of the device (100) allows substituting a usual Littrow and Littman-Metcalf configuration, with their usual known limitations and issues with regard to misalignments, for a set of diffractive lenses (2, 6, 8) , and replace also the rotated etalon for a tunable filter (4) , allowing therefore to the volume and the overall weight of the laser device (100) down to a fraction of the bulky one and also reduce product costs.

This allows to significantly reduce the size of the device (100) from tens of cm scale to millimiters' scale, avoiding expensive precision mechanic components and using bulky metal parts for thermal stabilization, while achieving the similar narrow linewidth and tunability range performances. An example of the possible cavity resonance scenarios is depicted in Figure 4 for a laser diode ( 1 ) emitting a light wavelength of 1550 nm .

The spectrum ( 204 ) represented in Figure 4 illustrates the modes that are fed back into the laser cavity .

One of the modes is higher than the other ones and represents the emission mode as , after multiple cycles of optical feedback, it will be amplified with higher gain than the other modes thus becoming the dominant resonating mode .

These spectra were obtained assuming a single wavelength round trip in the tunable filter ( i . e . , half wavelength cavity length) but it can be designed to be sharper , simply by setting the cavity length equal to multiple wavelengths . This can be achieved at the expenses of having a lower free spectral range , which does not constitute a problem as long as it is larger than the width of the laser medium gain, usually in the order of a few tens of nanometres .

A possible variation of this scheme involves the use of two hyper chromatic lenses ( 12 ) , axially displaced with MEMS devices and a pinhole ( 11 ) instead of a tunable Fabry-Perot filter ( 4 ) .