Chirped laser source device

Abstract

The invention provides a chirped laser source device for providing a chirped laser beam having a chirp rate that is of at least 20 MHz / V. In accordance with preferred embodiments, the achievable chirp rate may be much higher. As such, the proposed device enables a variety of applications that benefit from a laser beam having an elevated chirp rate and therefore a large spectral width. In particular, the proposed chirped laser source device allows to produce laser beams that enable CRBS analysis of air or of gases having very high temperatures, which is not achievable with known chirped laser sources.

Description

CHIRPED LASER SOURCE DEVICE

[0001] The present invention lies in the field of laser technology. In particular, the invention relates to a chirped laser source device.Background of the invention

[0002] Known chirped laser sources have been proposed for a variety of applications, for example in techniques such as Coherent Rayleigh and Brillouin Scattering, CRBS, for measuring properties of gases and liquids, such as sound speed and temperature. Such techniques are for example described in “Single-shot coherent Rayleigh-Brillouin scattering using a chirped optical lattice” Optics Letters Vol. 38, No. 21, November 1, 2013. A chirped waveform is generally considered to be a signal in which the frequency changes with respect to time. It has been observed that a laser beam with higher chirp rate and larger spectral width allows for the analysis of lighter and / or hotter particles. However, known laser sources are generally limited to chirp rates of around 10-15 MHz / V. Such chirp rates do not allow for the analysis of air or gases having temperatures higher than 1000K, for example.Technical problem to be solved

[0003] It is an objective to present method and device, which overcome at least some of the disadvantages of the prior art.Summary of the invention

[0004] In accordance with a first aspect of the invention, a chirped laser source device is proposed. The chirped laser source device comprises laser beam emitting means and a laser cavity arranged downstream of said laser beam emitting means, so that a path of an emitted laser beam passes through an electro-optical modulator element held inside the laser cavity. The laser cavity comprises electrical contact means for applying an electrical potential difference to opposite faces of the electro-optical modulator element. The chirped laser source device is remarkable in that a connection interface between the electro-optical modulator element and said electrical contact means comprises an electrically conductive filler material. It is understood that in the term “laser cavity” is used in what precedes to designate a cavity for holding the electro-optical modulator element.

[0005] In accordance with a further aspect of the invention, a chirped laser source device comprising laser beam emitting means is provided. The laser beam emitting means comprise an excitation device configured to cause a laser crystal to emit a laser beam and a collimating lens. The chirped laser source device further comprises a cavity, or equivalently: holding means, arranged downstream of said laser beam emitting means, so that a path of an emitted laser beam passes through an electro-optical modulator element held inside the cavity. The chirped laser source device also comprises output coupling means arranged downstream of the cavity on the path of an emitted laser beam, wherein the cavity comprises electrical contact means for applying an electrical potential difference to opposite faces of the electro-optical modulator element. The chirped laser source device is remarkable in that a connection interface between the electro-optical modulator element and said electrical contact means comprises an electrically conductive filler material.

[0006] The electrical contact means may preferably comprise copper electrodes.

[0007] Preferably, the electrically conductive filler material may comprise a malleable metal.

[0008] The electrically conductive filler material may preferably comprise a metal foil.

[0009] It may further be preferred that the electrically conductive filler material comprises Indium.

[0010] Preferably, the electro-optical modulator element may comprise a LiTaO3crystal having an electrically conductive coating on planar opposite faces for contacting said connection interface.

[0011] The chirped laser source device may preferably further comprise electric signal generation means connected to said electrical contact means, configured to apply a voltage signal to the electro-optical modulator element through said connection interface. The electric signal generation means may preferably comprise a voltage generator.

[0012] The laser beam emitting means may preferably comprise a laser crystal that is arranged upstream of the laser cavity, i.e., the cavity holding the electro-optical modulator element.

[0013] The laser beam emitting means may further preferably comprise an excitation device configured for exciting the laser crystal, causing the latter to emit said laser beam.

[0014] Preferably, the excitation device may comprise a laser diode having a first wavelength, a current source, or a light source. Downstream of the laser diode the laser beam emitting means may preferably comprise the collimating lens.

[0015] The chirped laser source device may preferably further comprise output coupling means arranged downstream of the laser cavity, i.e. the cavity holding the electro-optical modulation element, on the path of an emitted laser beam.

[0016] Preferably, the laser crystal may comprise a coating facing the laser diode, which is antireflecting at the first wavelength and highly reflective at a second, higher wavelength. The output coupling means may preferably comprise a coating facing the laser diode, which is partially reflective at the second wavelength.

[0017] In accordance with a second aspect of the invention, a method for producing a chirped laser beam using the chirped laser source device according to aspects of the invention is provided. The method comprises the steps of:

[0018] - causing the laser beam emission means to emit a laser beam;

[0019] - causing an electrical potential difference to be applied to opposite faces of the electro-optical modulator element through the connection interface.

[0020] Preferably, the step of causing an electrical potential difference to be applied to opposite faces of the electro-optical modulator element through the connection interface may comprise applying a voltage signal to opposite faces of the electro-optical modulator element through the connection interface, the voltage signal comprising a voltage sweep in the range of + / -50 V to + / - 400V.

[0021] In accordance with a further aspect of the invention, the use of a chirped laser source device according to aspects of the invention for producing a laser beam having a chirp rate of at least 10, at least 15, at least 20 or at least 25 MHz / V is provided.

[0022] According to yet another aspect of the invention, the use of a chirped laser source device according to aspects of the inventio for producing a laser beam having a chirp rate of at least 1 GHz / V is provided.

[0023] By using the proposed invention, it becomes possible to provide a chirped laser beam having a chirp rate that is of at least 20 MHz / V. In accordance with preferred embodiments, the achievable chirp rate may be higher than 25 MHz / V and even of the order of 1 GHz / V or higher. As such, the proposed device enables a variety of applications that benefit from a laser beam having an elevated chirp rate and therefore a large spectral width. In particular, the proposed chirped laser source device allows to produce laser beams that enable CRBS analysis of air or of gases having very high temperatures, which is not achievable with known chirped laser sources. The use of a malleable electrical interface between an electrode and an electro-optical modulator of the laser source device has been identified as a key feature for enhancing the electric contact between these elements. An improved electrical contact in turn allows for the application of higher voltage sweeps to the electro-optical modulator of the laser source, leading to higher chirp rates and increased spectral width of the produced laser beam.Brief description of the drawings

[0024] Several embodiments of the present invention are illustrated by way of figures, which do not limit the scope of the invention, wherein:

[0025] -provides a schematic illustration of a chirped laser source device in accordance with a preferred embodiment of the invention, including a cut-through representation of a cavity holding an electro-optical modulator element;

[0026] -provide a schematic front view of a cavity as used in a chirped laser source device in accordance with a preferred embodiment of the invention;

[0027] -illustrates the effect of using an improved electrical interface between electrodes and an electro-optical modulator as provided in a chirped laser source device in accordance with a preferred embodiment of the invention;

[0028] -illustrates the electrical connection shown inin accordance with a prior art embodiment, the improved electrical interface as provided with embodiments of the invention being absent;

[0029] -provides a schematic illustration of a chirped laser source device in accordance with a preferred embodiment of the invention, including a cut-through representation of a cavity;

[0030] -provides a basic workflow diagram showing the main steps of a method in accordance with a preferred embodiment of the invention.Detailed description of the invention

[0031] This section describes aspects of the invention in further detail based on preferred embodiments and on the figures. The figures do not limit the scope of the invention. Throughout the description, like numerals will be used to describe like concepts in different embodiments. Details that are described in the context of a particular embodiment are applicable to other embodiments, unless otherwise stated.

[0032] illustrates a laser source device 100 in accordance with a preferred embodiment of the invention. The laser source device 100 comprises laser beam emitting means 110 for producing a laser beam that follows a propagation path 112 indicated from left to right by a dashed line. The path 112 passes through a cavity 120 in which an electro-optical modulator element 130, EOM, is arranged. The cavity 120 comprises electrical contact means 122, 124, for example copper electrodes, for applying an electrical potential difference to opposite faces, i.e., the top and bottom faces, of the electro-optical modulator element 130. By changing the electrical potential difference that is applied to the electro-optical modulator element 130, the laser beam passing therethrough is provided with a different modulation frequency. Thus, sweeping a voltage range on the EOM 130 induces a chirp modulation on the passing laser beam. The connection interface 140 between the electrical contacts 122, 124 and the electro-optical modulator element 130 comprises an electrically conductive filler material 142. As shown, the conductive filler material 142 is preferably arranged between the top surface of the EOM 130 and a first electrical contact 122, and between the bottom surface of the EOM 130 and a second electrical contact 124, respectively. Whileshows a cut-through view of the cavity 120,provides a front view of the cavity 120 including the electrical contacts 122, 124, the improved connection interface 140, the electrically conductive filler material 142 and the electro-optical modulator element 130, as seen in the direction of the laser beam 112.

[0033] In accordance with preferred embodiments of the invention, the electro-optical modulator element 130 is a LiTaO3crystal having an electrically conductive coating on planar opposite faces for contacting the electrically conductive filler material 142. The coating of the EOM may preferably by a gold plating. In preferred embodiments of the invention, the electrically conductive filler material comprises a malleable metal, which is resilient to deformations. The metal may for example be Indium, and it may preferably be provided as a foil, the dimensions of the filler material 142 being exaggerated in figures 1 and 2 for the sake of clarity only.

[0034] The effect of the filler material is illustrated in. The dimensions and shape of the electro-optical modulator element 130 are generally well defined. However, due to machining and construction constraints during production of a laser source device, the orientation of the contacting electrodes 122, 124 may happen to become askew, as shown by way of example. If the planar electrodes end up being in any orientation that is not entirely coplanar to the plated surface of the EOM 130, the filler material 140 allows correction of this geometric flaw by re-establishing the electrical contact between substantially the entire plated surface of the EOM 130 and the respective electrodes 122, 124. This allows for more current to pass through the EOM 130 as the resistance of the electrical connection between the electrodes 122, 124 and the EOM is reduced. By way of reference, the same configuration without the filler material 142 is illustrated in: this is the arrangement that is observed in known chirped laser sources. As the electrical contact between the EOM 130 and the electrodes 122, 124 is impaired by the geometrical arrangement of the electrodes, the modulating properties of the EOM are inherently limited.

[0035] shows another preferred embodiment of the chirped laser source 200 in accordance with the invention, wherein further details are explained. While this embodiment groups several preferred features, such as preferred laser beam emitting means 210 comprising a laser crystal 250, a voltage generator 260 and output coupling means 270, intermediate embodiments wherein only one of these features are incorporated, or wherein intermediate combinations thereof are incorporated, are herewith also implicitly disclosed and within the scope of the present invention. Preferably, all elements of the laser source device 200 are installed on non-illustrated thermal management means, such as a block of aluminum, which allows to dissipate the heat produced by the device.

[0036] The laser source device 200 comprises laser beam emitting means 210 including a pumping diode 214, a laser crystal 250 and a collimating lens 216 or similar optical means, for producing a laser beam, that follows a propagation path 212 indicated from left to right by a dashed line.

[0037] The pump laser diode 214 and collimating lens 216 are configured to provide energy to the laser crystal 250. The laser diode has a first wavelength, for example 808nm, which matches an energy level of the laser crystal 250.

[0038] After traversing the collimating lens 216, the path 212 passes through the laser crystal 250, such as an Nd:YVO4crystal. Alternatively, the laser crystal may be excited by a different energy source, such as a flashlight source or an electrical current source, instead of the laser diode 214. Such means are as such known in the art.

[0039] The laser crystal 250 comprises a coating on the diode-facing side. On, this corresponds to the left-hand side of the laser crystal. The coating is preferably an embedded coating, which is antireflecting (AR) at the first wavelength, e.g. 808nm to allow for the pump diode to hit it inside. The coating is highly reflective (HR) at a second wavelength, e.g. 1064nm. Practically, by absorbing the first wavelength, 808nm, the laser crystal is forced to emit at the second wavelength, 1064 nm. Preferably, the laser crystal 250 comprises an antireflecting coating at the second wavelength on its second face, facing away from the laser diode, in the direction of the output.

[0040] Further downstream, the path 212 then passes through a cavity 220 in which an electro-optical modulator element 230, EOM, is arranged. Preferably the length of the cavity along the direction of the laser beam is of about 5mm, and the EOM preferably has about the same length. The cavity comprises electrical contact means 222, 224, for example copper electrodes, for applying an electrical potential difference to opposite faces, i.e., the top and bottom faces, of the electro-optical modulator element. By changing the electrical potential difference that is applied to the electro-optical modulator element 230, the laser beam passing therethrough is provided with a different modulation frequency. Thus, sweeping a voltage range on the EOM 230 induces a chirp modulation on the passing laser beam. A voltage generator 260 provides electric signal generation means. The generator 260 is connected to the electrical contacts 222, 224, and it is configured to apply a voltage signal to the electro-optical modulator element 230. The signal may be linear, sinusoidal, or of any shape, as required by the desired chirp modulation of the laser beam, which in turn depends on the final application in which the laser beam is used. The connection interface 240 between the electrical contacts 222, 224 and the electro-optical modulator element 230 comprises an electrically conductive filler material 242, such as a malleable Indium foil. As shown, the conductive filler material 242 is preferably arranged between the top surface of the EOM 230 and a first electrical contact 222, and between the bottom surface of the EOM 230 and a second electrical contact 224, respectively. After passing through the cavity 220, the path 212 further passes through an output coupler element 270.

[0041] The output coupler 270 (OC) is partially reflective (PR) at the second wavelength 1064 nm. This means that it allows some light having the second wavelength, e.g. 1064 nm light out, thereby providing the laser output, while the remaining radiation circulates between the OC and the HR side of the laser crystal 250.

[0042] The distance, L, between the HR side of the laser crystal 250 and the output coupling means 270 multiplied by the refractive index n (n=1 for air) is called the optical pathlength and this is the distance light effectively travels within the medium.

[0043] The frequency output of the cavity formed between the laser crystal 250 and 270, including nothing else, i.e. in absence of an electro-optical modulator element, would be given by f=c / (2nL) where c is the speed of light.

[0044] However, the presence of an electro-optical modulator crystal, EOM, 230 having two gold electrodes changes the propagation medium of the laser beam. The EOM has its own refractive index, n, which means that by introducing within the cavity, we change the optical pathlength of the cavity and thus the frequency output, f.

[0045] The refractive index n of the EOM changes when a voltage is applied to the EOM. This results in a chirped laser cavity, as the frequency of the laser cavity may be changed as a function of time, by applying a corresponding voltage signal to the EOM crystal.

[0046] illustrates a summary of the method steps for producing a chirped laser beam using the device that has been described by way of several embodiments. The voltage signal applied to the EOM my span the range from -50 to +50V, or larger potential difference leading up to -400 to +400V, without limiting the invention to these examples. Lower sweeping ranges induce lower chirp rates, for example of the order of 20 MHz / V, while larger sweeping ranges allow to produce chirp rates of 1 GHz / V or even higher.

[0047] It should be noted that features described for a specific embodiment described herein may be combined with the features of other embodiments unless the contrary is explicitly mentioned.

[0048] It should be understood that the detailed description of specific preferred embodiments is given by way of illustration only, since various changes and modifications within the scope of the invention will be apparent to the person skilled in the art. The scope of protection is defined by the following set of claims.

Claims

A chirped laser source device (100, 200) comprising laser beam emitting means (110, 210), the laser beam emitting means comprising an excitation device (214) configured to cause a laser crystal (250) to emit a laser beam and a collimating lens (217), a cavity (120, 220) arranged downstream of said laser beam emitting means, so that a path of an emitted laser beam (112, 212) passes through an electro-optical modulator element (130, 230) held inside the cavity, and output coupling means (270) arranged downstream of the cavity on the path of an emitted laser beam, wherein the cavity comprises electrical contact means (122,124 ; 222,224) for applying an electrical potential difference to opposite faces of the electro-optical modulator element (130, 230),characterized in that a connection interface (140, 240) between the electro-optical modulator element and said electrical contact means comprises an electrically conductive filler material (142, 242).The chirped laser source device (100, 200) according to claim 1, wherein the electrically conductive filler material (142, 242) comprises a malleable metal.The chirped laser source device (100, 200) according to any of the preceding claims, wherein the electrically conductive filler material (142, 242) comprises a metal foil.The chirped laser source device (100, 200) according to any of the preceding claims, wherein the electrically conductive filler material (142, 242) comprises Indium.The chirped laser source device (100, 200) according to any of the preceding claims, wherein the electro-optical modulator element (130, 230) comprises a LiTaO3crystal having an electrically conductive coating on planar opposite faces for contacting said connection interface (140, 240).The chirped laser source device (200) according to any of the preceding claims, further comprising electric signal generation means (260) connected to said electrical contact means (222, 224), configured to apply a voltage signal to the electro-optical modulator element (230) through said connection interface (240).The chirped laser source device (200) according to any of the preceding claims, wherein the excitation device comprises a laser diode (214) having a first wavelength.The chirped laser source device (200) according to claim 7, wherein the laser crystal (250) comprises a coating facing the laser diode (214), which is antireflecting at the first wavelength and highly reflective at a second, higher wavelength, and wherein the output coupling means (270) comprise a coating facing the laser diode (214), which is partially reflective at the second wavelength.A method for producing a chirped laser beam using the chirped laser source device according to any of claims 1 to 8, comprising the steps of:- causing the laser beam emitting means (110, 210) to emit a laser beam;- causing an electrical potential difference to be applied to opposite faces of the electro-optical modulator element (130, 230) through the connection interface (140, 240).The method in accordance with claim 9, wherein causing an electrical potential difference to be applied to opposite faces of the electro-optical modulator element through the connection interface comprises applying a voltage signal to opposite faces of the electro-optical modulator element through the connection interface, the voltage signal comprising a voltage sweep in the range of + / -50 V to + / - 400V.Use of a chirped laser source device according to any of claims 1 to 8 for producing a laser beam having a chirp rate of at least 25 MHz / V.Use of a chirped laser source device according to any of claims 1 to 8 for producing a laser beam having a chirp rate of at least 1 GHz / V.

Images