Tunable laser

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CW dye laser based on Rhodamine 6G. The dye laser is considered to be the first broadly tunable laser. Coherent 899 dye laser.jpg
CW dye laser based on Rhodamine 6G. The dye laser is considered to be the first broadly tunable laser.

A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.

Contents

There are many types and categories of tunable lasers. They exist in the gas, liquid, and solid state. Among the types of tunable lasers are excimer lasers, gas lasers (such as CO2 and He-Ne lasers), dye lasers (liquid and solid state), transition metal solid-state lasers, semiconductor crystal and diode lasers, and free electron lasers. [1] Tunable lasers find applications in spectroscopy, [2] photochemistry, atomic vapor laser isotope separation, [3] [4] and optical communications.

Types of tunability

Single line tuning

Since no real laser is truly monochromatic, all lasers can emit light over some range of frequencies, known as the linewidth of the laser transition. In most lasers, this linewidth is quite narrow (for example, the 1,064 nm wavelength transition of a Nd:YAG laser has a linewidth of approximately 120 GHz, or 0.45 nm [5] ). Tuning of the laser output across this range can be achieved by placing wavelength-selective optical elements (such as an etalon) into the laser's optical cavity, to provide selection of a particular longitudinal mode of the cavity.

Multi-line tuning

Most laser gain media have a number of transition wavelengths on which laser operation can be achieved. For example, as well as the principal 1,064 nm output line, Nd:YAG has weaker transitions at wavelengths of 1,052 nm, 1,074 nm, 1,112 nm, 1,319 nm, and a number of other lines. [6] Usually, these lines do not operate unless the gain of the strongest transition is suppressed; e.g., by use of wavelength-selective dielectric mirrors. If a dispersive element, such as a prism, is introduced into the optical cavity, tilting of the cavity's mirrors can cause tuning of the laser as it "hops" between different laser lines. Such schemes are common in argon-ion lasers, allowing tuning of the laser to a number of lines from the ultraviolet and blue through to green wavelengths.

Narrowband tuning

For some types of lasers the laser's cavity length can be modified, and thus they can be continuously tuned over a significant wavelength range. Distributed feedback (DFB) semiconductor lasers and vertical cavity surface emitting lasers (VCSELs) use periodic distributed Bragg reflector (DBR) structures to form the mirrors of the optical cavity. If the temperature of the laser is changed, the index change of the DBR structure causes a shift in its peak reflective wavelength and thus the wavelength of the laser. The tuning range of such lasers is typically a few nanometres, up to a maximum of approximately 6 nm, as the laser temperature is changed over ~50 K. As a rule of thumb the wavelength is tuned by 0.08 nm/K for DFB lasers operating in the 1,550 nm wavelength regime. Such lasers are commonly used in optical communications applications such as DWDM-systems to allow adjustment of the signal wavelength. To get wideband tuning using this technique, some such as Santur Corporation or Nippon Telegraph and Telephone (NTT Corporation) [7] contain an array of such lasers on a single chip and concatenate the tuning ranges.

Widely tunable lasers

A typical laser diode. When mounted with external optics these lasers can be tuned mainly in the red and near infrared. Diode laser.jpg
A typical laser diode. When mounted with external optics these lasers can be tuned mainly in the red and near infrared.

Sample Grating Distributed Bragg Reflector lasers (SG-DBR) have a much larger tunable range, by the use of vernier tunable Bragg mirrors and a phase section, a single mode output range of >50 nm can be selected. Other technologies to achieve wide tuning ranges for DWDM-systems [8] are:

Rather than placing the resonator mirrors at the edges of the device, the mirrors in a VCSEL are located on the top and bottom of the semiconductor material. Somewhat confusingly, these mirrors are typically DBR devices. This arrangement causes light to ‘‘bounce’’ vertically in a laser chip, so that the light emerges through the top of the device, rather than the edge. As a result, VCSELs produce beams of a more circular nature than their cousins and beams that do not diverge as rapidly. [10]

As of December 2008 there is no widely tunable VCSEL commercially available any more for DWDM-system application.[ citation needed ]

It is claimed that the first infrared laser with a tunability of more than one octave was a germanium crystal laser. [11]

Applications

The range of applications of tunable lasers is extremely wide. When coupled to the right filter, a tunable source can be tuned over a few hundreds of nanometers [12] [13] [14] with a spectral resolution that can go from 4 nm to 0,3 nm, depending on the wavelength range. With a good enough isolation (>OD4) tunable source can be used for basic absorption and photoluminescence study. It can be used for solar cells characterisation in a light beam induced current (LBIC) experiment from which external quantum efficiency (EQE) of a device can be mapped. [15] It can also be used for the characterisation of gold nanoparticles [16] and single-walled carbon nanotube thermopile [17] where a wide tunable range from 400 nm to 1,000 nm is essential. Tunable sources were recently used for the development of hyperspectral imaging for early detection of retinal diseases where a wide range of wavelength, a small bandwidth and outstanding isolation is crucial in order to achieve an efficient illumination of the entire retina. [18] [19] Tunable source can be a powerful tool for reflection and transmission spectroscopy, photobiology, detector calibration, hyperspectral imaging and steady-state pump probe experiment to name only a few.

History

The first true broadly tunable laser was the dye laser in 1966. [20] [21] Hänsch introduced the first narrow-linewidth tunable laser in 1972. [22] Dye lasers and some vibronic solid-state lasers have extremely large bandwidths, allowing tuning over a range of tens to hundreds of nanometres. [23] Titanium-doped sapphire is the most common tunable solid-state laser, capable of laser operation from 670 nm to 1,100 nm wavelength. [24] Typically these laser systems incorporate a Lyot filter into the laser cavity, which is rotated to tune the laser. Other tuning techniques involve diffraction gratings, prisms, etalons, and combinations of these. [25] Multiple-prism grating arrangements, in several configurations, as described by Duarte, are used in diode, dye, gas, and other tunable lasers. [26]

See also

Related Research Articles

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<span class="mw-page-title-main">Laser diode</span> Semiconductor laser

A laser diode is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction.

<span class="mw-page-title-main">Dye laser</span> Equipment using an organic dye to emit coherent light

A dye laser is a laser that uses an organic dye as the lasing medium, usually as a liquid solution. Compared to gases and most solid state lasing media, a dye can usually be used for a much wider range of wavelengths, often spanning 50 to 100 nanometers or more. The wide bandwidth makes them particularly suitable for tunable lasers and pulsed lasers. The dye rhodamine 6G, for example, can be tuned from 635 nm (orangish-red) to 560 nm (greenish-yellow), and produce pulses as short as 16 femtoseconds. Moreover, the dye can be replaced by another type in order to generate an even broader range of wavelengths with the same laser, from the near-infrared to the near-ultraviolet, although this usually requires replacing other optical components in the laser as well, such as dielectric mirrors or pump lasers.

The term biophotonics denotes a combination of biology and photonics, with photonics being the science and technology of generation, manipulation, and detection of photons, quantum units of light. Photonics is related to electronics and photons. Photons play a central role in information technologies, such as fiber optics, the way electrons do in electronics.

<span class="mw-page-title-main">Vertical-cavity surface-emitting laser</span> Type of semiconductor laser diode

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<span class="mw-page-title-main">Distributed Bragg reflector</span> Structure used in waveguides

A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with different refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection and refraction of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the interaction between these beams generates constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is "forbidden" to propagate in the structure.

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<span class="mw-page-title-main">Prism compressor</span> Optical device

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A distributed-feedback laser (DFB) is a type of laser diode, quantum-cascade laser or optical-fiber laser where the active region of the device contains a periodically structured element or diffraction grating. The structure builds a one-dimensional interference grating, and the grating provides optical feedback for the laser. This longitudinal diffraction grating has periodic changes in refractive index that cause reflection back into the cavity. The periodic change can be either in the real part of the refractive index or in the imaginary part. The strongest grating operates in the first order, where the periodicity is one-half wave, and the light is reflected backwards. DFB lasers tend to be much more stable than Fabry–Perot or DBR lasers and are used frequently when clean single-mode operation is needed, especially in high-speed fiber-optic telecommunications. Semiconductor DFB lasers in the lowest loss window of optical fibers at about 1.55 μm wavelength, amplified by erbium-doped fiber amplifiers (EDFAs), dominate the long-distance communication market, while DFB lasers in the lowest dispersion window at 1.3 μm are used at shorter distances.

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<span class="mw-page-title-main">Solid-state dye laser</span>

A solid-state dye laser (SSDL) is a solid-state lasers in which the gain medium is a laser dye-doped organic matrix such as poly(methyl methacrylate) (PMMA), rather than a liquid solution of the dye. These lasers are also referred to as solid-state organic lasers and solid-state dye-doped polymer lasers.

<span class="mw-page-title-main">Multiple-prism grating laser oscillator</span>

Multiple-prism grating laser oscillators, or MPG laser oscillators, use multiple-prism beam expansion to illuminate a diffraction grating mounted either in Littrow configuration or grazing-incidence configuration. Originally, these narrow-linewidth tunable dispersive oscillators were introduced as multiple-prism Littrow (MPL) grating oscillators, or hybrid multiple-prism near-grazing-incidence (HMPGI) grating cavities, in organic dye lasers. However, these designs were quickly adopted for other types of lasers such as gas lasers, diode lasers, and more recently fiber lasers.

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<span class="mw-page-title-main">Distributed Bragg reflector laser</span>

A distributed Bragg reflector laser (DBR) is a type of single frequency laser diode. Other practical types of single frequency laser diodes include DFB lasers and external cavity diode lasers. A fourth type, the cleaved-coupled-cavity laser has not proven to be commercially viable. VCSELs are also single frequency devices. The DBR laser structure is fabricated with surface features that define a monolithic, single mode ridge waveguide that runs the entire length of the device. A resonant cavity is defined by a highly reflective DBR mirror on one end, and a low reflectivity cleaved exit facet on the other end. Within the cavity is a gain ridge portion, where current is injected to produce a single spatial lasing mode. The DBR mirror is designed to reflect only a single longitudinal mode. As a result, the laser operates on a single spatial and longitudinal mode. The laser emits from the exit facet opposite the DBR end. The DBR is continuously tunable over approximately a 2 nm range by changing current or temperature. The temperature coefficient is approximately 0.07 nm/K, and the current coefficient is approximately 0.003 nm/mA. DBR lasers are stable, low noise optical sources. When operated with a low noise power supply at constant temperature, edge emitting DBR lasers have a linewidth of less than 10 MHz. Power levels typically can run up to several hundred milliwatts.

References

  1. F. J. Duarte (ed.), Tunable Lasers Handbook (Academic, 1995).
  2. W. Demtröder, Laser Spectroscopy: Basic Principles, 4th Ed. (Springer, Berlin, 2008).
  3. J. R. Murray, in Laser Spectroscopy and its Applications, L. J. Radziemski, R. W. Solarz, and J. A. Paisner (Eds.) (Marcel Dekker, New York, 1987) Chapter 2.
  4. M. A. Akerman, Dye-laser isotope separation, in Dye Laser Principles, F. J. Duarte and L. W. Hillman, Eds. (Academic, New York, 1990) Chapter 9.
  5. Koechner, §2.3.1, p49.
  6. Koechner, §2.3.1, p53.
  7. Tsuzuki, K.; Shibata, Y.; Kikuchi, N.; Ishikawa, M.; Yasui, T.; Ishii, H.; Yasaka, H. (2009). "Full C-Band Tunable DFB Laser Array Copackaged with InP Mach–Zehnder Modulator for DWDM Optical Communication Systems". IEEE Journal of Selected Topics in Quantum Electronics. 15 (3): 521–527. Bibcode:2009IJSTQ..15..521T. doi:10.1109/jstqe.2009.2013972. S2CID   27207596.
  8. Tunable Lasers at Lightreading
  9. P. Zorabedian, Tunable external-cavity semiconductor lasers, in Tunable Lasers Handbook, F. J. Duarte, Ed. (Academic, New York, 1995) Chapter 8.
  10. "Optoelectronics, Frequency Changing". studedu.org. Retrieved 2024-03-07.
  11. See photograph 3 at http://spie.org/x39922.xml
  12. PhotonEtc: Tunable Laser Source from 400nm to 2300nm.
  13. Leukos : White light compact supercontinuum systems.
  14. Fianium : Powerful WhiteLase Supercontinuum Sources.
  15. L. Lombez; et al. (2014). "Micrometric investigation of external quantum efficiency in microcrystalline CuInGa(S,Se)2 solar cells". Thin Solid Films. 565: 32–36. Bibcode:2014TSF...565...32L. doi:10.1016/j.tsf.2014.06.041.
  16. S. Patskovsky; et al. (2014). "Wide-field hyperspectral 3D imaging of functionalized gold nanoparticles targeting cancer cells by reflected light microscopy". Journal of Biophotonics. 8 (5): 401–407. doi:10.1002/jbio.201400025. PMID   24961507. S2CID   6797985.
  17. St-Antoine B, et al. (2011). "Single-Walled Carbon Nanotube Thermopile For Broadband Light Detection". Nano Letters. 11 (2): 609–613. Bibcode:2011NanoL..11..609S. doi:10.1021/nl1036947. PMID   21189022.
  18. Shahidi AM, et al. (2013). "Regional variation in human retinal vessel oxygen saturation". Exp Eye Res. 113: 143–7. doi:10.1016/j.exer.2013.06.001. PMID   23791637.
  19. Tunable Lasers For Retinal Imaging.
  20. F. P. Schäfer (ed.), Dye Lasers (Springer, 1990)
  21. F. J. Duarte and L. W. Hillman (eds.), Dye Laser Principles (Academic, 1990)
  22. Hänsch, T. W. (1972). "Repetitively Pulsed Tunable Dye Laser for High Resolution Spectroscopy". Appl. Opt. 11 (4): 895–898. Bibcode:1972ApOpt..11..895H. doi:10.1364/ao.11.000895. PMID   20119064.
  23. Koechner, §2.5, pp66–78.
  24. Steele, T. R.; Gerstenberger, D. C.; Drobshoff, A.; Wallace, R. W. (1991). "Broadly tunable high-power operation of an all-solid-state titanium-doped sapphire laser system". Optics Letters. 16 (6): 399–401. Bibcode:1991OptL...16..399S. doi:10.1364/OL.16.000399. PMID   19773946.
  25. F. J. Duarte and L. W. Hillman (eds.), Dye Laser Principles (Academic, 1990) Chapter 4
  26. F. J. Duarte, Tunable Laser Optics, 2nd Ed. (CRC, New York, 2015) Chapter 7.

Further reading