JP2010153836A - Littman configured frequency stepped laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a frequency stepped laser system having a modified Littman configuration, and to provide a method using this system. <P>SOLUTION: A wavelength tunable system includes: a laser having a laser resonator 104 and an external resonator 110 which have a common optical axis; a reflective grating 214 fixed in the external resonator 110; a collimator lens 112 between the laser resonator 104 and the reflective grating 214; and an adjustable reflection mirror 220 defining one end of the external resonator 110, optically coupled to the fixed reflective grating 214 and pivotally rotating around the position of the fixed reflective grating. The utilization method thereof is also provided. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  This application claims the benefit of US Provisional Application No. 61 / 118,162, filed Nov. 26, 2008.

  The present invention relates to wavelength tunable laser systems, and more particularly to frequency step laser systems and methods.

  In this embodiment, the present disclosure provides a frequency step laser system having a modified Littman configuration and a method of using the system.

1 is a schematic diagram of a known Littrow configuration frequency step external cavity tunable laser. FIG. Modifications in embodiments of the present disclosure that have reflective mirrors oriented at different pivot angles with respect to a fixed reflective diffraction grating and produce corresponding different laser wavelengths within the bandwidth of an external cavity tunable laser FIG. 3 is a schematic diagram of a frequency step-external cavity tunable laser with a Littman configuration. FIG. 4 is a second schematic diagram of a modified Littman configuration frequency step external cavity tunable laser in an embodiment of the present disclosure. FIG. 4 illustrates exemplary results of proper mode matching between an external resonator and a basic laser diode resonator in an embodiment of the present disclosure. FIG. 5 is a perspective view of an exemplary implementation of a Littman step laser in an embodiment of the present disclosure.

Detailed Description of the Invention

  Various embodiments of the present invention will be described in detail with reference to the drawings, if any. Reference to various embodiments does not limit the scope of the invention, but only by the claims appended hereto. Moreover, any example set forth in this specification describes only a few of the many possible embodiments for the claimed invention.

[Definition]
"Wavelength tunable" or similar terms relate to a laser light source, for example, where the instantaneous frequency or wavelength output can be controlled to change.

  “Frequency step” or similar terms relate to a laser light source, for example, where the instantaneous frequency or wavelength output can be one of a discrete number of possible frequencies or wavelengths.

  "Including" or like terms refers to including or including, but not exclusive or limiting.

  “About” is used in describing embodiments of the present disclosure to modify components, portions or aspects of the present disclosure, such as angles, distances, thicknesses and similar values and ranges thereof, such as apparatus And typical measurement and handling procedures used to manufacture or use the parts, accidental errors in these procedures, differences in manufacturing or source of materials or components used to perform the method, and the like This refers to changes in quantity that can occur in matters. Where the term “about” is used in the appended claims, it includes equivalents thereof.

  “Essentially composed” of embodiments, for example, relates to a frequency step laser apparatus and system having a modified Littman configuration and a method of making or using the system as recited in the claims. May include elements or steps, as well as other elements or steps that do not substantially affect the basic and novel characteristics of the disclosed items, apparatus or systems, and methods of making or using, for example, certain configurations It may include elements, specific materials, specific configurations, specific conditions, or similar structures, materials or selected process variables. Items or aspects that substantially affect the basic characteristics of components, configurations or steps of the present disclosure or that provide undesirable characteristics to the present disclosure include, for example, the relative non-pivoting (rotation) of optical elements Includes movement or lateral displacement.

  Accordingly, the claimed invention may comprise, consist of, or consist essentially of, for example, an apparatus or system as defined herein, or a method of making and using such a system as defined herein. It is configured.

  As used herein, the indefinite article "a" or "an" and its corresponding definite article "the" means at least one, or one or more, unless specified otherwise.

  Abbreviations well known to those skilled in the art (eg, “h” or “hr” for time or hours, “nm” for nanometers, “μ” for micron, and similar abbreviations) may be used.

  The disclosed specific and preferred values for components, configurations, directions and similar aspects, and ranges thereof, are for illustration only and exclude other defined values or values within other defined ranges. do not do. The devices, systems, and methods of the present disclosure include any value described herein, or a specific value, and any combination of specific values and preferred values.

  Tunable laser systems are known, for example, WO200006112971 entitled “MODE-MATCHING SYSTEM FOR TUNABLE EXTERNAL CAVITY LASER”, “Mode Selective”. US Pat. No. 7,209,499 entitled “Mode-Selective Frequency Tuning System”, “Phase-Resolved Measurement for Frequency Shifting Interferometry” US Pat. No. 7,268,889 entitled “)”, US Pat. No. 7,259,860 entitled “Optical Feedback from Mode-Selective Tuner”, and “Adjustment” Tunable laser system (“Tunable La” with possible external cavity See US Pat. No. 6,690,690 entitled “Ser System having an Adjustable External Cavity”).

  The above-mentioned patent refers to a tunable laser system incorporating, for example, a standard uncoated laser diode coupled to an external resonator having a Littrow configuration grating. The angle of the diffraction grating can be adjusted to change the wavelength of the light fed back to the laser diode. The diffraction grating can be pivoted about an axis in the same plane as the plane of the diffraction grating, the pivot axis being parallel to the ruling direction of the diffraction grating surface. The length of the external cavity is selected to be mode matched to the basic laser diode. As the grating angle is continuously changed, the output beam frequency of the laser changes in discrete frequency steps called mode hops. In ideally mode-matched conditions, the mode hop or frequency step is determined by the length of the fundamental laser diode and its optical refractive index. The above-mentioned patent mentions a frequency step laser system having a Littrow resonator design and its use.

  There are tunable laser systems that allow “continuous tuning” without mode hops, using different tuning methods. These tunable laser systems use laser diodes where one of the diode end faces has a special anti-reflective (AR) coating. The mode structure of the laser is determined by the length of the external cavity, and there are no competing modes from the laser diode itself. The length of the external resonator is changed while changing the angle of the diffraction grating, and the same mode is used over the tuning band. This continuous tuning technique is not very suitable for comb frequency tuning. There are applications where a set of steps with a constant frequency interval is desired, for example a step of 35 GHz, for example in the range of 30 nm band from 820 nm to 850 nm. Continuous tuning techniques include at least two basic, including, for example, that anti-reflective coatings can be difficult to control manufacturing and quality, and that small diffraction grating angular changes can cause undesirable frequency changes. Has drawbacks.

  Frequency step lasers using a Littrow configuration are disclosed, for example, in US Pat. No. 7,209,499 and references cited therein. The Littrow configuration external cavity tunable laser uses a rotationally adjustable diffraction grating in which the primary reflection from the diffraction grating is returned into the laser cavity.

  External cavity tunable lasers having a Littmann or Littrow configuration are known, for example, “Wavelength Tunable Light Source Integrated with Optical Amplifier, Beam Steering Unit”. , and Concave Diffraction Grating ")" and U.S. Pat. No. 7,133,194 and FIGS. However, these configurations attempt to obtain continuous tuning by tracking the same resonator mode across the tuning band. This requires controlling the change in external resonator length during tuning. On the other hand, according to the present disclosure, the frequency step tuning is performed by fixing the external cavity length and mode-matching with the basic laser diode.

  Existing continuous tunable lasers require an excellent anti-reflective coating on the base laser diode exit end face. Non-reflective coatings can be expensive, have reduced laser life and have failed in the field. These tunable lasers also have certain inherent limitations, for example in applications that require repeated scanning over a predetermined set of frequencies. Because these lasers are “continuously tunable”, small angular errors can cause significant frequency errors. This limitation requires that a significant amount of scanning time be spent “stabilizing” to minimize frequency errors. This reduces the throughput.

  We already have a tunable laser technology that removes these limitations using a frequency stepping design where the external cavity tunable laser length is fixed or held constant while the laser is tuned across the entire band. Reported. See, for example, WO2006112971, US 7,209,499, US 7,268,889, US 7,259,860 and US 6,690,690. The frequency step lasers of these patent documents use a Littrow configuration in which the diffraction grating can be pivotally adjusted, and the angle can be adjusted to return the first-order backward reflection in the laser resonator.

In an embodiment, the present disclosure is a wavelength tunable system comprising:
A laser having a common optical axis and a laser having an external cavity;
A reflective diffraction grating fixed in the external resonator;
A collimator lens between the laser resonator and the reflective diffraction grating;
An adjustable reflective mirror defining one end of the external resonator and optically coupled to the fixed reflective diffraction grating, wherein the adjustment pivots about the position of the fixed reflective diffraction grating A wavelength tunable system having a possible reflection mirror.

  In an embodiment, the adjustable reflection mirror pivotably adjusts about one axis, the axis being co-planar with the plane of the reflective diffraction grating. . The pivot axis may be parallel to a ruling on the plane of the diffraction grating. The adjustable reflecting mirror can be pivotally adjustable about the axis of the fixed reflective diffraction grating. The wavelength of the laser output beam can be changed discretely by sequentially changing the reflection mirror to different angles. In embodiments, the laser may be, for example, an optical amplifier. The length of the external resonator may be constant, for example. The primary reflection may be incident on an adjustable reflection mirror, for example. The direction of the zero-order reflection may be invariant, for example, and may not be incident on the adjustable reflection mirror, for example.

In embodiments, the present disclosure provides:
Providing the above tunable system;
Sequentially or sequentially orienting an adjustable reflective mirror to each of the N pivot position angles (pivot rotation angles) θN with respect to the reflective diffraction grating and outputting a beam having each of a series of wavelengths λN. A method for tuning the wavelength of a light source is provided.

In an embodiment, the present disclosure is a method for wavelength tuning of a laser, for example used in an interrogation (detection) biosensor device, comprising:
Providing the above tunable system;
Orienting the tunable reflecting mirror in a series of sequential pivot angles θN with respect to the reflective grating and outputting a beam having each of a series of wavelengths λN;
Irradiating the beam towards a grating-based waveguide biosensor;
Collecting the radiation reflected from the biosensor and measuring the relative intensity of the reflected radiation at each of the different wavelengths λN;
Analyzing the collected intensity data against wavelength to determine a maximum peak wavelength response of the biosensor.

In an embodiment, the present disclosure is a method of wavelength tuning a laser comprising:
Providing the above tunable system;
Sequentially or sequentially orienting a tunable reflecting mirror at each of N pivot angles θN with respect to a fixed reflecting grating and outputting a beam having each of a series of wavelengths λN. Provide a method to make.

  The “N” may be an integer including, for example, 2 to about 10,000, 2 to about 200, 2 to about 100, and all values and ranges therebetween. The angle θN includes, for example, about 1 degree to about 5 degrees, about 2 degrees to about 4 degrees, and all values and ranges therebetween, for example, about 3 degrees.

  In embodiments, the system of the present disclosure can be used with or incorporated into a stand-alone laser, or other product or device, for example, for measurement applications, optical sensing such as biosensing, medical, communications and similar applications, etc. More complex or integrated systems can be constructed.

  The apparatus and method of the present disclosure is superior to known systems and methods, for example, in that it provides lower cost, easier manufacturing and better performance in manufacturing and ownership. In an embodiment, the present disclosure provides a frequency step laser system having a Littman configuration. A laser system having a Littman configuration has several advantages over the Littrow laser configuration described above and as described below.

  In embodiments, the present disclosure provides a method, laser apparatus and system for achieving frequency step scanning by using an external cavity tunable laser in a Littman configuration. An external cavity tunable laser with a Littman configuration uses a reflective optical system such as a controllable pivoting reflecting mirror that angularly tunes the primary reflection from a reflective diffraction grating that is positionally fixed in the lateral and rotational directions. An important aspect of the disclosed system and method is that the disclosed angular tuning does not produce a zero order beam that changes direction.

  In an embodiment, the present disclosure provides a Littman frequency step laser having a fixed diffraction grating and an adjustable mirror that retroreflects the primary reflection into the laser resonator. The disclosed system provides several advantages. Since the diffraction grating is fixed, the zero order beam is also fixed angularly, and the zero order beam does not move during angle tuning. To date, engineers have had to include a mirror that moves with the diffraction grating to minimize the movement of the zero order beam. Having a fixed output beam provides cost savings, for example, by simplifying the mechanical configuration of the system and eliminating problems normally associated with fiber coupling of the output beam. There are also some adjustments to the diffraction grating that are typically made during the laser assembly. These adjustments are significantly simplified because they can be separated from the angle adjustment design.

  In embodiments, the present disclosure provides an apparatus and method with frequency step tuning of a modified Littman configuration external cavity tunable laser. Referring to the drawings, FIG. 1 is a schematic diagram of a known external cavity tunable laser for frequency step tuning of a Littrow configuration (100) laser. See, for example, US Pat. No. 7,209,499. As shown, the diffraction grating rotates about the pivot axis and the resonator length is fixed while the laser is tuned over the operating band.

  In both configurations of FIGS. 1 and 2, a standard uncoated laser diode (LD) (102) with an internal resonator (104) is connected to an external resonator (110) by a collimator lens or similar dispersive element. Is bound to. The external resonator (110) or (118) includes the distance between the fixed diffraction grating and the pivoting mirror. In the prior art Littrow configuration of FIG. 1, a rotatable diffractive optical element such as a diffraction grating (114) further having a rotation axis (115) and a diffraction grating angle θ (116) has a first order reflection (118) of a laser beam. Adjusted back into the diode. The diffraction grating (114) can rotate about an axis (115). The axis of rotation (115) is coplanar with the plane of the diffraction grating, is parallel to the score lines of the plane of the diffraction grating, and is perpendicular to the incident and reflected beams. By slightly changing the grating angle θ, the wavelength of the light fed back into the laser diode can be changed, thus changing the laser wavelength output light (Po) (125). Fixed reflective optical elements, such as the mirror (120), undergo zero order reflection (122).

  A partial schematic diagram of the laser apparatus (200) and system of the present disclosure is shown in FIG. 2, and with respect to the fixed reflective diffraction grating (214), for example, different (three of 220a, 220b, 220c are shown) pivots. It has a controllable pivoting mirror (220) oriented in the corners and produces corresponding different laser wavelengths within the external cavity tunable laser band. The pivot point (215) for the reflective mirror (220) can be selected, for example, to be on the plane of the reflective grating (214), and the external resonator length can be constant over the tuning band. In the modified Littman configuration of the present disclosure of FIG. 2, the tuning angle θ (216) of the reflective mirror (220) is imaginary or actual located in the reflective diffraction grating (214) fixed with respect to rotation and pivoting (rotation). Can be pivotally adjusted around a remote location (215), such as a point or pivot axis, and the primary reflection (118) is returned toward the internal cavity of the laser diode (104). By applying a slight pivot rotation change to the reflecting mirror angle θ (216), eg, about 200 steps over a pivot range of about 3 degrees, the wavelength of the light fed back into the laser diode is changed and the laser wavelength is stepped. Change. The mirror (220) can be pivoted about a remote pivot axis (215) that is, for example, coplanar with the plane of the fixed grating and parallel to the score line of the plane of the grating.

  FIG. 3 is a partial schematic diagram of the system (300) of the present disclosure having a Littman configuration frequency step and external cavity laser. In this configuration, the reflection diffraction grating (214) is held at a fixed angle, and the primary reflection is incident on the reflection mirror (220) that can be pivotally adjusted. The adjustable reflective mirror is changed to change the angle of the reflected beam (318). This changes the laser wavelength of the tunable laser. The position of the pivot point of the reflective mirror (220) at or on the surface of the reflective grating (214) keeps the external cavity length (110) constant while the laser is tuned over the operating band. Retained.

  The purpose of the configurations of FIGS. 2 and 3 is to keep the external cavity length (110) constant when the laser is angled over the operating band. The fixed length of the external resonator (110) is selected to be mode matched to the basic laser diode (102). An example of mode matching (mode matching) for a basic laser diode is schematically shown in FIG.

  FIG. 4 shows exemplary results of proper mode matching between an external resonator and a basic laser diode resonator. The laser diode mode spacing is an integer multiple of the external cavity mode. In the illustrated example (scale not shown), the optical path length of the external resonator is selected to be 5 times (5X) the optical path length of the fundamental laser diode mode. The spacing of the fundamental laser diode resonator mode (410) is an integer multiple of the spacing of the external cavity longitudinal mode (420). As the tuning angle (216) or mirror angle θ is continuously changed, the laser frequency changes in discrete frequency steps called mode hops. In ideal mode matching conditions, the mode hop or frequency step is determined by the length of the fundamental laser diode (104) and its optical refractive index.

  In either Littrow or Littman configuration, mode matching requires that the main or fundamental laser diode mode spacing is a multiple of the external cavity mode spacing to achieve frequency step tuning.

[Example]
The examples below serve to more fully describe the method of using the present disclosure above, and illustrate examples of the best mode for carrying out various aspects of the present disclosure. These examples are not intended to limit the scope of the present disclosure, but are presented for illustrative purposes.

[Example 1]
Numerous modifications and changes can be used to implement the devices and methods of the present disclosure. An example of the device configuration (500) is shown in the perspective view of FIG. A pivotable mirror (505) is mounted on a mirror support plate (510) that is remotely attached to a rotating axle assembly (515). The axle (515) is positioned such that the axis of rotation (520) is mechanically aligned with the plane of the fixed diffraction grating (525), for example, and parallel to the grooves on the fixed diffraction grating (525). The diffraction grating (525) is fixed and all of its mechanical adjustment can be performed independently of the rotating axle. The lever arm (530) is attached to the axle and can be moved, for example, by a linear motor (535). A laser beam (540) from an external cavity can be directed to a fixed diffraction grating (525) and a pivotable mirror (505). The linear motor may be of any suitable type or model. In embodiments, the linear motor may be a stepper motor, a voice coil assembly, a piezoelectric actuator or a DC motor, for example. The moving range of the motor may depend on the length of the lever arm, for example. The length of the arm may be optimized to match the performance of the selected motor. For example, a small voice coil assembly that moves over a 6 mm range and a 100 mm arm can change the mirror angle over about 3 degrees.

The present disclosure has been described in terms of various specific embodiments and techniques. However, various modifications and changes are possible within the scope of the present disclosure.

Claims (5)

A wavelength tunable system,
A laser having a common optical axis and a laser having an external cavity;
A reflective diffraction grating fixed in the external resonator;
A collimator lens between the laser resonator and the reflective diffraction grating;
An adjustable reflective mirror defining one end of the external resonator and optically coupled to the fixed reflective diffraction grating, wherein the adjustment pivots about the position of the fixed reflective diffraction grating A wavelength tunable system, comprising:   The wavelength tunable of claim 1, wherein the adjustable reflective mirror pivotably adjusts about one axis, the axis being coplanar with the surface of the reflective diffraction grating. ·system.   3. The wavelength tunable system according to claim 2, wherein the pivot axis of rotation is parallel to a score line on the diffraction grating surface. A method for tuning the wavelength of a laser,
Providing the system of claim 1;
Orienting the adjustable reflecting mirror at each of N pivot angles θN with respect to the reflective diffraction grating sequentially or sequentially to output a beam having each of a series of wavelengths λN. Method. A method of tuning the wavelength of a laser in an interrogation biosensor device,
Orienting the tunable reflecting mirror of the tunable system of claim 1 to a series of sequential pivot angles θN with respect to the reflective grating and outputting a beam having each of a series of wavelengths λN; ,
Irradiating the beam towards a grating-based waveguide biosensor;
Collecting the radiation reflected from the biosensor and measuring the relative intensity of the reflected radiation at each of the different wavelengths λN;
Analyzing the collected intensity data against wavelength to determine a maximum peak wavelength response of the biosensor.

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