Principles of Mode-Hop Free Wavelength Tuning

Transcription

1 Principles of Mode-Hop Free Wavelength Tuning Table of Contents 1. Introduction Tunable Diode Lasers in Littrow Cavity Design Pivot Point Requirements Realization according to Nilsson & Vilhelmsson (True Pivot Point) Realization according to Ricci & Hänsch (Virtual Pivot Point) Realization according to Sacher (Virtual Pivot Point) Dual Pivot Point Realization (Virtual & Fixed Pivot Point) Realization according to Sacher (Virtual & Fixed Pivot Point) Tunable Diode Lasers in Littman/Metcalf Cavity Design Pivot Point Requirements Realization according to Luecke, Sacher (True Pivot Point) Realization according to Sacher (True Pivot Point) Alignment Insensitive Cavity Design Summary and Conclusions Littrow Type of Diode Laser Cavities Littman / Metcalf Type of Diode Laser Cavities Literature Tunable diode lasers are a valuable tool for simple and easy wavelength tuning. Depending on the application, there are different quality requirements on the tuning performance of tunable diode lasers. For simple applications, wavelength tuning with discontinuous tuning performance which is referred to as mode-hops are acceptable. Demanding applications require mode-hop free tuning performance. Document: Page: 1

2 1. Introduction Principles of Tunable Diode Lasers: A typical tunable diode laser system consists of a laser cavity and a wavelength selective element, e.g. a diffraction grating, like shown in Fig. 1. The diffraction grating acts as an intra-cavity spectral filter for the laser emission. Grating Laser Figure 1: Schematic setups of a tunable laser systems with grating feedback. Methods of Changing the Laser Wavelength. There are several ways of changing the laser wavelength. (a) Changing the angle of incidence of the cavity grating. (b) Changing the cavity length. (c) Combination of both methods. The three methods are shown schematically in Fig. 2. (a) Wavelength Tuning via Rotating the Diffraction Grating: Rotation of the grating will result into a change of the spectral position of the filter curve of the grating as shown in Fig. 2 (a). It does not result into a continuous change of the wavelength, as this is knows from non-lasing light sources. The reason for this is as follows. Lasers generate standing waves within the laser cavity which are commonly referred to as laser modes or cavity modes. Document: Page: 2

3 (a) Grating (b) Grating (c) Grating Figure 2: Spectral view of the laser modes defined by the laser cavity and the filter which is defined by the diffraction grating of the laser cavity show in Fig. 1. (a) Rotation of the grating without a change of the cavity length. (b) Change of the cavity length without a rotation of the grating. (c) Synchronous rotation of the grating and change of the cavity length. Document: Page: 3

4 The wavelength of the laser modes is defined by the optical length of the cavity. Rotating the grating around a rotation axis which is located in the surface of the grating, will only result into a change of the filter curve of the grating. The wavelength which is defined by the length of the cavity (cavity modes) will not change. Only the laser mode which is favored by the grating will be lasing. In summary, rotating the grating will result into a discontinuous wavelength change. The laser will change wavelength in discrete steps which are defined by the distance of the laser modes. (b) Wavelength Tuning via Changing the Laser Cavity Length: Changing the length of the laser cavity will result into a change of the spectral position of the laser modes, as shown in Fig. 2 (b). Within a small tuning range the laser wavelength will follow the change of the laser mode. A soon as a neighbor mode coincides better with the filter function of the grating, the laser will change the emission mode and the laser wavelength changes discontinuously. By changing the length of the laser cavity, a saw tooth like of wavelength tuning will occur. (c) Wavelength Tuning via a Combined Changing of the Laser Cavity Length and the Grating Angle: For mode hop-free wavelength tuning of lasers, it is required to perform a change of the grating angle a synchronously with a change of the cavity length. There are several technical realizations of tunable diode lasers which offer mode hop free tuning, which will be discussed in the following sections of this document. Document: Page: 4

5 2. Tunable Diode Lasers in Littrow Cavity Design A very common type of tunable diode lasers follows the design of spectrometers. They are commonly referred to Littrow type of lasers. 2.1 Pivot Point Requirements The Pivot Point requirements for mode-hop free tuning of Littrow type of external cavity diodes have been analyzed by McNicholl and Metcalf in 1985 [1]. Figure 3: The schematics shows the schematic of a Littrow cavity according to [1]. The Pivot Point is indicated as A. X 0 is the distance of the cavity mirror to the Pivot Point A. X 1 is the distance of the grating to the Pivot Point A. McNichol and Metcalf performed an analysis on the requirements for achieving mode-hop free tuning with Littrow types of cavities. According to their analysis, mode-hop free tuning can be achieved with a proper choice of the Pivot Point A. For achieving this, the following condition needs to be fulfilled: X 0 = X 1 = 0 (1) with: X 0: Distance between Cavity Mirror and Pivot Point A X 1: Distance between Diffraction Grating and Pivot Point A Document: Page: 5

6 2.2 Realization according to Nilsson & Vilhelmsson (True Pivot Point) Figure 4: The schematics shows the schematic of a Littrow cavity according to [2]. Nilson and Vilhelson applied this condition determined by McNicholl and Littman to Diode Lasers with Exteral Cavity in Littrow Configuration [2]. They took the optical path length within the laser diode into account and first introduced the concept of the Virtual Pivot Point, where the Pivot Point is located at a dynamic position not coinciding with the bending point of a flex-mount tuning design. According to their approach, one of the first fully mode-hop free tuning external cavity diode laser systems was realized as early as Sacher Lasertechnik was supporting this development with the supply of antireflection coated laser diodes. Document: Page: 6

7 2.3 Realization according to Ricci & Hänsch (Virtual Pivot Point) The Ricci Hänsch realization [3] of the Littrow type of tunable diode lasers chooses a Virtual Pivot Point with X 1, X 2 =/= 0 unequal to zero. Pivot Point Grating Laser Figure 5: Schematic view of the Ricci Hänsch realization of a Littrow type of laser tunable diode laser [3]. The left hand side shows the technical realization. The right hand side shows an analysis of this realization according to the choice of Pivot Point. X 0 indicates the distance between the back mirror of the laser diode to the Pivot Point. X 1 indicates the distance of the grating plane to the Pivot Point. This realization results into a mechanically very stable solution. The violation of the Pivot Point Requirement Eq. (1) reduces the mode hop-free tuning range to typically 3-8 GHz. Since the Pivot Point changes dynamically with the grating angle, the Ricci Hänsch type of design is one of the first known realizations of a Virtual Pivot Point. The Virtual Pivot Point enables mode hop-free wavelength scans of typically 8 GHz which is larger than expected for the choice of X 0, X 1. A synchronous modulation of the laser diode current, even slightly larger mode-hop free tuning values may be achieved of up to 30GHz. Drawback of this method of enhancing the mode-hop free tuning range is a significant change of laser power during the wavelength scan. The Ricci Hänsch type of External Cavity Laser in Littrow design is optimized as a tunable fixed wavelength laser for investigating atomic transitions of alkaline metals with great stability features. Large wavelength scans are difficult to achieve with the Ricci Hänsch design due to the limited bending range of the flex mount design of the grating holder. The Ricci Hänsch type of External Cavity Diode Laser in Littrow Design is a tunable fixed wavelength type of laser. Document: Page: 7

8 2.4 Realization according to Sacher (Virtual Pivot Point) The Ricci Hänsch type Littrow Cavity causes a high power power density at the laser facet. For example, a 20mW emitting laser system shows an estimated power of 25mW emitting and 4mW returning from the grating which results into a total power of 29mW at the laser facet. Pivot Point Grating Laser Figure 6: Schematic view of the Sacher realization of a Littrow type of laser tunable diode laser [4]. X 0 indicates the distance between the back mirror of the laser diode to the Pivot Point. X 1 indicates the distance of the grating plane to the Pivot Point. The laser power is emitted from the rear facet of the laser diode. The Sacher Littrow realization [4] overcomes the problem of high power at the laser facet. The outcoupling laser facet is performed with a special high power durable type of facet coating. The damage threshold of the facet coating is between 500mW and 1000mW for a 1µm x 3µm laser emitter. The cavity laser facet shows only low power values in the order of 3-5mW which ensures a long duration of the laser diode. The Sacher Littrow realization shows comparable features as the classical Ricci Hänsch type of Littrow cavity. The Pivot Point is realized as a Virtual Pivot Point. This design enables mode hopfree wavelength scans of typically 8 GHz which is larger than expected for the choice of X 0, X 1. A synchronous modulation of the laser diode current, even slightly larger mode-hop free tuning values may be achieved of up to 30GHz. Drawback of this method of enhancing the mode-hop free tuning range is a significant change of laser power during the wavelength scan. The Sacher Littrow type of External Cavity Laser is optimized as a tunable fixed wavelength laser for investigating atomic transitions of alkaline metals with great stability features. Large wavelength scans are difficult to achieve with the Sacher Littrow design due to the limited bending range of the flex mount design of the grating holder. The Sacher Littrow type of External Cavity Diode Laser is a tunable fixed wavelength type of laser. Document: Page: 8

9 2.5 Dual Pivot Point Realization (Virtual & Fixed Pivot Point) For overcoming the problems with large wavelength scans of the Ricci Hänsch type of design, the Virtual Pivot Point concept was developed. Fig. 6 shows a schematic layout of this concept [5]. Pivot Point 2 Pivot Point 1 Grating Laser Rotation Figure 7: Schematic view of a Littrow type laser tunable diode laser with virtual Pivot Point [5]. The grating is mounted into a rotation stage with the grating in the center of the rotation stage. The grating holder is realized as a flex-mount type of holder. Pivot Point 1: The grating holder is positioned within a rotational stage. The Pivot Point of the rotational stage is referred to as Pivot Point 1. Pivot Point 1 does not fulfill the condition (1) for mode-hop free tuning. Therefore, wavelength tuning according to Pivot Point 1 does only result into a 3-5GHz mode-hop free tuning range. The mode-hop occurs as soon as one free spectral range of laser cavity is passed. Pivot Point 2: The grating holder positioned within the rotational stage allows a fine wavelength tuning with a Virtual Pivot Point type of solution. Tuning according Pivot Point 2 results into mode hop-free tuning range to typically 8GHz, comparable to the Ricci Hänsch type of Littrow Cavity design. A synchronous modulation of the laser diode current, even slightly larger mode-hop free tuning values may be achieved of up to 30GHz. Drawback of this method of enhancing the mode-hop free tuning range is a significant change of laser power during the wavelength scan. The Dual Pivot Point (Virtual & Fixed) type of design is optimized as a tunable fixed wavelength laser for investigating atomic transitions of alkaline metals with great stability features. Document: Page: 9

10 2.6 Realization according to Sacher (Virtual & Fixed Pivot Point) The Dual Pivot Point Littrow Cavity causes a high power power density at the laser facet. For example, a 20mW emitting laser system shows an estimated power of 25mW emitting and 4mW returning from the grating which results into a total power of 29mW at the laser facet. Pivot Point 2 Pivot Point 1 Grating Laser Rotation Figure 8: Schematic view of a Littrow type laser tunable diode laser with virtual Pivot Point. The grating is mounted into a rotation stage with the grating in the center of the rotation stage. The grating holder is realized as a flex-mount type of holder. The Sacher Littrow realization [4] overcomes the problem of high power at the laser facet. The outcoupling laser facet is performed with a special high power durable type of facet coating. The damage threshold of the facet coating is between 500mW and 1000mW for a 1µm x 3µm laser emitter. The cavity laser facet shows only low power values in the order of 3-5mW which ensures a long duration of the laser diode. Pivot Point 1: The grating holder is positioned within a rotational stage. The Pivot Point of the rotational stage is referred to as Pivot Point 1. Pivot Point 1 does not fulfill the condition (1) for mode-hop free tuning. Therefore, wavelength tuning according to Pivot Point 1 does only result into a 3-5GHz mode-hop free tuning range. The mode-hop occurs as soon as one free spectral range of laser cavity is passed. Pivot Point 2: The grating holder positioned within the rotational stage allows a fine wavelength tuning with a Virtual Pivot Point type of solution. Tuning according Pivot Point 2 results into mode hop-free tuning range to typically 8GHz, comparable to the Ricci Hänsch type of Littrow Cavity design. Document: Page: 10

11 A synchronous modulation of the laser diode current, even slightly larger mode-hop free tuning values may be achieved of up to 30GHz. Drawback of this method of enhancing the mode-hop free tuning range is a significant change of laser power during the wavelength scan. The Dual Pivot Point (Virtual & Fixed) type of design is optimized as a tunable fixed wavelength laser for investigating atomic transitions of alkaline metals with great stability features. Document: Page: 11

12 3. Tunable Diode Lasers in Littman/Metcalf Cavity Design A second type of tunable diode lasers bases on cavity designs with a grazing incidence of the laser light to a grating. They are commonly referred to Littman/Metcalf type of lasers. 3.1 Pivot Point Requirements The Pivot Point requirements for mode-hop free tuning of Littman/Metcalf type of external cavity diodes have been analyzed by Liu and Littman in 1981 [4] and McNicholl and Littman in 1985 [1]. Figure 9: The schematics shows the schematic of a Littrow cavity according to [1]. The Pivot Point is indicated as A. X 0 is the distance of the cavity mirror to the Pivot Point A. X 1 is the distance of the grating to the Pivot Point A. X 2 is the distance of the tuning mirror to the Pivot Point A. The schematic on the left hand side show the general situation. The schematic on the right hand side shows the most common type of realization. McNichol and Littman performed an analysis on the requirements for achieving mode-hop free tuning with Gracing Incidence types of cavities [6], [1]. According to their analysis, mode-hop free tuning can be achieved with a proper choice of the Pivot Point A. For achieving this, the following condition needs to be fulfilled: X 0 = X 1 = X 2 = 0 (2) with: X 0: Distance between Cavity Mirror and Pivot Point A X 1: Distance between Diffraction Grating and Pivot Point A Distance between Tuning Mirror and Pivot Point A X 2: Document: Page: 12

13 3.2 Realization according to Luecke, Sacher (True Pivot Point) The Luecke realization of the Littman/Metcalf type of tunable diode lasers chooses a True Pivot Point with proper choice of X 0, X 1, X 2 =/= 0 for realizing a large mode-hop free tuning range [7]. Figure 10: Schematic view of a Littman/Metcalf type laser tunable diode laser. X 0 indicates the distance between the back mirror of the laser diode to the Pivot Point. X 1 indicates the distance of the grating plane to the Pivot Point. X 2 indicates the distance between the tuning mirror of the laser diode to the Pivot Point. Large mode hop-free tuning of Littman/Metcalf type of tunable lasers can be achieved by a proper choice of rotation axis of the grating. Fully Mode hop-free tuning is achieved for the following values: X 0 = 0, X 1 = 0, X 2 = 0, or an equivalent choice [7, 8]. This realization results into a mechanically very stable solution. Fulfilling the equivalent to the Pivot Point Requirement Eq. (2) results into a large mode hop-free tuning range of more than 100GHz. Motorized versions of the Littman/Metcalf type of cavity result into fully mode-hop tuning over the entire tuning range of the laser diodes. The Littman / Metcalf type of External Cavity Diode Laser is optimized for large mode-hop free wavelength scans. Typical applications are investigations of Quantum Dot semiconductor structures, micro cavities, photonic crystals and much more. The Littman / Metcalf type of External Cavity Diode Laser is a true tunable type of tunable diode laser. Document: Page: 13

14 3.3 Realization according to Sacher (True Pivot Point) The Luecke type Littman/Cavity Cavity [4] causes a high power power density at the laser facet. For example, a 20mW emitting laser system shows an estimated power of 40mW emitting and 10mW returning from the grating which results into a total power of 50mW at the laser facet. Figure 11: Schematic view of a Littman/Metcalf type laser tunable diode laser. X 0 indicates the distance between the back mirror of the laser diode to the Pivot Point. X 1 indicates the distance of the grating plane to the Pivot Point. X 2 indicates the distance between the tuning mirror of the laser diode to the Pivot Point [8, 9]. The Sacher Littman / Metcalf realization [8, 9] overcomes the problem of high power at the laser facet. The outcoupling laser facet is performed with a special high power durable type of facet coating. The damage threshold of the facet coating is between 500mW and 1000mW for a 1µm x 3µm laser emitter. The cavity laser facet shows only low power values in the order of 3-5mW which ensures a long duration of the laser diode. The Sacher realization of the Littman/Metcalf type of tunable diode lasers chooses a True Pivot Point with proper choice of X 0, X 1, X 2 =/= 0 for realizing a large mode-hop free tuning range. The Sacher Littman / Metcalf realization shows comparable features as the Luecke type of Littman / Metcalf cavity. This realization results into a mechanically very stable solution. Fulfilling the equivalent to the Pivot Point Requirement Eq. (2) results into a large mode hop-free tuning range of more than 100GHz. Motorized versions of the Sacher Littman / Metcalf type of cavity result into fully mode-hop tuning over the entire tuning range of the laser diodes. The Sacher Littman / Metcalf type of External Cavity Diode Laser is optimized for large mode-hop free wavelength scans. Typical applications are investigations of Quantum Dot semiconductor structures, micro cavities, photonic crystals and much more. The Sacher Littman / Metcalf type of External Cavity Diode Laser is a true tunable type of tunable diode laser. Document: Page: 14

15 3.4 Alignment Insensitive Cavity Design Figure 12: Schematic view of the Roof Prism 15 of Littman/Metcalf type laser tunable diode laser. 61 indicates the incoming beam, 62 the outcoming beam [8]. Due to the right angle 154 of the roof prism, beam 62 is always parallel to beam 61, independent of the angle of incidence. Technical realizations of of Littman/Metcalf type of tunable lasers show large mode-hop free tuning ranges, even for large wavelength scans. Replacing the Tuning Mirror with a Roof Prism results into a cavity design which is tolerant against mechanic disturbances [8]. The special features of the roof prism as described in Fig. 12 ensure a perfect alignment under all conditions. The great features of the roof prism design ensure large mode-hop free wavelength scans, even when the tuning mirror is moved via a motor. The roof prism design is optimized for applications which require large mode-hop free wavelength scans as molecular spectroscopy, quantum dot spectroscopy, or interferometry. Document: Page: 15

16 4. Summary and Conclusions A brief insight into the design and typical optimizations of Littrow type and Littman / Metcalf type of tunable diode lasers was provided. Typical optimizations of Littrow and Littman / Metcalf cavity design are: 4.1 Littrow Type of Diode Laser Cavities Littrow type of External Cavity Diode Lasers are designed and optimized as a tunable fixed wavelength laser for investigating atomic transitions of alkaline metals with great stability features. Typical mode-hop free tuning ranges without applying additional features for enhancing the modehop free tuning range are in the order of 8GHz. A synchronous modulation of the laser diode current, even slightly larger mode-hop free tuning values may be achieved of up to 30GHz. Drawback of this method of enhancing the mode-hop free tuning range is a significant change of laser power during the wavelength scan. Large wavelength scans are difficult to achieve with the Ricci Hänsch design due to the limited bending range of the flex mount design of the grating holder. 4.2 Littman / Metcalf Type of Diode Laser Cavities Littman / Metcalf type of External Cavity Diode Lasers are designed and optimized as a true tunable laser with large mode-hop free wavelength scans. Typical applications are investigations of Quantum Dot semiconductor structures, micro cavities, photonic crystals and much more. Large mode hop-free tuning of Littman/Metcalf type of tunable lasers can be achieved by a proper choice of rotation axis of the tuning mirrorfully Mode hop-free tuning of more than 100GHz is achieved for a proper choice of the Pivot point. Large wavelength scans are achieved with motorized versions of the Littman/Metcalf type of cavity which result into fully mode-hop tuning over the entire tuning range of the laser diodes. Document: Page: 16

17 5. Literature [1] Patrick McNicholl, Harold Metcalf, Synchronous cavity mode and feedback wavelength scanning in dye laser oscillators with gratings, Applied Optics 24 (17), page , Sept [2] Olle Nilsson, Kenneth Vilhelmsson, Method for ascertaining mode hopping free tuning of resonance frequency and the Q-value of an optical resonator and a device for carrying out the method, WO 91/03848, Sept [3] L. Ricci, M. Weidemüller, T. Esslinger, A. Hemmerich, C. Zimmermann, V. Vuletic, W. König, T.W. Hänsch, A compact grating-stabilized diode laser system for atomic physics, Optics Communications 117, page , June 1995 [4] Joachim Sacher, Laserdioden Anordnung mit externem Resonator, EP , Nov [5] Thomas Heine, Rainer Heidemann, Abstimmbares Diodenlasersystem mit externem Resonator, EP , June 2007 [6] Karen Liu, Michael Littman, Novel geometry for single-scanning of tunable lasers, Optics Letters 6 (3), page , March 1981 [7] Francis Luecke, Tuning system for external cavity diode laser, US , Sept [8] Joachim Sacher, Abstimmvorrichtung, EP , April 1996 [9] S. Stry, S. Thelen, J. Sacher, D. Halmer, P. Hering, M. Mürtz, Widely tunable diffraction limited 1000mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy, Applied Physics B85, page , 2006 Document: Page: 17