Patent Application
20190393671
The disclosure relates generally to frequency conversion of laser beams, and in particular to a solid state Raman-active medium for generating laser beams at the desired frequencies. In particular, the present disclosure relates to a selectable multi-wavelength solid state Raman laser system, a method for selectively providing an output laser beam from the laser system at different wavelengths and applications based on the disclosed system and method.
The lasers available today, especially tunable devices, span a considerable range of wavelengths. Still, modern lasers are not capable of generating beams of sufficient quality at all desirable wavelengths. For this reason, frequency shifting or converting systems are used to derive the required frequencies. It is important that such systems produce high quality beams, i.e., beams of high intensity, and exhibit wavelength diversity and agility. In this case, wavelength diversity refers to the ability of producing laser beams at many different wavelengths, while wavelength agility is a measure of the maximum spread of wavelengths.
Techniques for shifting the wavelength of a laser beam are well known to one of ordinary skill in the laser arts. There are a number of elements which are capable of receiving a beam at a particular frequency and shifting it to a different frequency. These elements include Raman media. Stimulated Raman scattering (SRS), i.e., laser-stimulation of scattering in a Raman-active medium, is based on the Raman phenomenon briefly explained immediately below.
An intense laser beam incident on a molecular medium with internal degrees of freedom may be scattered by that medium in a variety of processes. In one of these, Raman scattering, following an inelastic collision a molecule is left in an excited state, and the scattered photon produced by interaction with that molecule will experience a wavelength shift in accordance with the principle of energy conservation. In the case where the molecule makes a transition to a higher energy level, the photon is scattered with a lower energy than it had when it arrived and thus has a longer wavelength than the incident light. This type of wavelength shift is called a Stokes shift. In the event where the molecular transition is to a lower energy level, the scattered photon carries away the excess energy, and thus has higher energy and longer wavelength than the incident light. The shifting to a longer wavelength is called an anti-Stokes shift.
Solid-state Raman lasers are a practical and efficient approach to optical frequency conversion, offering high (up to 70 to 80%) conversion efficiencies with respect to the pump power, excellent beam quality and ease of alignment. The use of crystals for SRS has been gaining interest because, in comparison, for example, with fiber Raman converters, crystalline Raman lasers offer greater gain increments, better thermal and mechanical properties, and significantly greater values of Raman shift. The greater gain coefficient leads to significant decrease of Raman conversion thresholds. The thermal and mechanical stability allow higher peak and average pulse pump powers and high pulse repetition frequency. The greater values of Raman shifts facilitate the concentration of energy at the desired Stokes and allow suppressing the parametric processes.
The Raman laser systems including solid-state Raman converters use many technological advances in the areas of light sources, fiber optic guides, spectrometers, medicine, red, green and blue (RGB) luminaire systems for digital cinema and etc, making Raman scattering a valuable tool in a variety of industrial, research and other applications. Many of these fields require the availability of different wavelengths.
In this regard, U.S. Pat. No. 4,165,469 teaches a solid-state laser capable of providing different frequencies of laser output light. Yet, the laser as taught, is limited to the use of lithium iodate crystal, which performs the functions of both Raman-shifter and frequency doubler to generate a plurality of possible output frequencies based on the frequency-doubled first, and then second or higher order Stokes stimulated Raman scattering. This limitation may be disadvantageous, as the disclosed converter is limited to the output frequencies obtainable using lithium iodate, since it is rare to find crystals capable of performing both of these functions together. Another disadvantage is that, since the lithium iodate crystal serves two discrete functions, it may not be possible to optimize the position of that crystal independently for two functions. Yet a further disadvantage is that lithium iodate has limited utility in high power applications, since it has a relatively low damage threshold.
Therefore a need exists for a tunable Raman laser system.
There is a further need for a tunable Raman laser system having a simple structure that can be deployed in the field without undue difficulty.
These needs are met by the disclosed structure and method teaching the arrangement of optical components that cumulatively allow increasing the length of optical path propagated by a desired Stokes wave frequency υd and a Stokes wave frequency υd−1 through a solid state Raman medium. The Stokes wave frequency acts as pump light for the desired wave frequency υd. For example, if the desired frequency is a first Stokes frequency υ1, then it is light from an external light source at a fundamental frequency υf that functions as pump light for the first Stokes in the Raman medium. The efficiency of Raman wavelength conversion depends on how long the waves at desired and pump frequencies are overlapped while propagating through the Raman medium. The efficiency increases with a longer overlap.
As light propagates through the Raman medium, the first Stokes that becomes pump light for a second Stokes wave frequency, which, in turn, becomes pump light for a third Stokes, etc. This process can continue over a wide range of Stokes frequencies. With the energy transfer between previous and subsequent Stokes, obviously the energy of the previous Stokes is depleted. Accordingly, while it is highly desired to create conditions for long interaction between the pump and desired pump frequencies, it is essential that a frequency higher than the desirable one be removed from the Raman medium as soon as possible. Thus on one hand, the optical interaction length through the Raman medium for fundamental and first Stokes wave frequencies is critical. On the other hand with a relatively great length, the firsts Stokes in the above-described example may generate a subsequent, second Stokes and thus loose its energy. Accordingly, the second Stokes should be filtered out from the Raman medium without undue delay.
In accordance with one aspect of the disclosure, a solid state Raman medium is configured with a crystal zigzagged by pump light between input and output of the crystal such that the pump light sequentially converts to Stokes wave frequencies υ1-υn. The Raman medium further is configured with spaced opposite sides bridging the input and output and a wavelength discriminator which is configured to guide the desired Stokes frequency to the exit of the Raman medium while being transparent to a Stokes wave frequency which is lower (i.e., longer wavelength) than the desired frequency. For example, if the first Stokes wave frequency υ1 is desired, then the second Stokes wave is filtered out along the light path through the Raman medium.
In accordance with another aspect of the disclosure, a Raman laser source includes an external laser pump operative to emit pump light at a fundamental frequency of along a path through the solid state Raman medium of the above aspect.
A further aspect of the disclosure that may be incorporated in the structure of any of the above aspects relates to the output of the Raman medium. It is configured to be transparent to the desired Stokes wave frequency, but reflects the remaining unconverted part of the pump light at the fundamental frequency back into the Raman medium towards the input. Returning to the example above, if the first Stokes is desired, then the pump light at fundamental frequency of is reflected back into the medium while the second Stokes is being filtered out. If the second Stokes is the desired frequency, the fundamental and first Stokes wave frequencies are reflected back into the Raman medium, while the third Stokes is being removed from the Raman medium upstream from the the exit.
In accordance with a further aspect, the wavelength discriminator of any of the above aspects is configured with a plurality of discreet reflectors. The reflectors include an inner layer facing the side of the Raman medium and one or more outer layers. The layers, from the inner one out, reflect respective fundamental frequency, Raman wave frequencies higher than the desired Raman wave frequency and desired Raman wave frequency back into the Raman medium.
According to another aspect of the disclosure, the wavelength discriminator of any of the above aspects the opposite sides of the Raman medium is coated with the discriminator. Alternatively, the wavelength discriminator of any of the above aspects is coated on one of the opposite sides of the Raman medium, but is spaced from the other side.
The solid state Raman medium of any of the above aspects is configured with a plurality of fast axis collimators arranged in a row between the discriminator and one of the opposite sides of the Raman medium. In addition, the solid state Raman medium further includes a slow axis collimator spaced downstream outside the output of the crystal.
In a further aspect of the disclosure, the solid state Raman medium of any of the above aspects can be an anisotropic, isotropic, uniaxial or biaxial crystal. The crystal may be selected from Ba(NO3)2, KGd(WO4)2, LiLo3, LiNbo3 or any other crystal considered to be a Raman-active medium.
In accordance with still another aspect of the disclosure, a Raman laser source is configured with an optical pump outputting pump light at a fundamental frequency along a linear path. The Raman source further includes a one piece upstream coupler configured to pass the pump light at the fundamental frequency, and a one piece downstream coupler configured to reflect the pump light. The upstream and downstream couplers define an outer optical cavity there-between and provide at least one round trip for the pump light within the outer cavity.
Furthermore, the Raman source in accordance with the previous aspect further has a one piece intermediate coupler spaced inwards from the upstream and downstream couplers and configured to be transparent to the pump light. Accordingly, the upstream and intermediate couplers define an inner optical cavity. The laser also includes a solid state Raman medium located within the inner optical cavity and configured to sequentially convert the pump light to Stokes wave frequencies υ1-υn of the fundamental frequency.
The one piece upstream coupler is partially transparent to pump light and is configured to focus the transmitted pump light into the Raman medium. The downstream face of the upstream coupler fully reflects Stokes waves. The one piece downstream coupler fully reflects the pump light and is fully transparent to generated Stokes wave frequencies. Finally, the one piece intermediary coupler has an upstream face partially reflecting the desired Stokes wave frequency back into the inner optical cavity while being fully transparent to frequencies higher than the desired frequency. The downstream face of the intermediary coupler is configured to collimate the pump and Stokes wave frequencies propagating towards the downstream coupler.
Another aspect of the disclosure relates to the laser pump of any of the above aspects. The laser pump is configured to operate in a continuous wave or pulsed regime. The laser pump outputs the pump light in single transverse mode or multiple transverse modes, polarized or unpolarized in a wavelength range varying from about 200 nm to about 2 μm. In a pulse regime, the fiber laser source may output pulses in a ps or longer temporal regimes.
In yet another aspect of the disclosure relates to any of the above aspects disclosing a pulsed operating regime of the fiber laser pump. In particular, the downstream coupler of the outer cavity is displaceable relative to the upstream coupler such that the reflected leading portion of forward propagating pulse of pump light overlaps the railing portion of this pulse which is still within the Raman medium. Accordingly, the inner cavity is constantly pumped.
The above and other configurations and features will become more readily apparent from the specific description in conjunction with the following drawings, in which:
A wavelength discriminator 22, coupled to opposite sides of Raman medium 18, is configured to guide a desired Stokes wave frequency υd along Raman medium 18 until the desired Stokes is decoupled from the Raman crystal through output 24. The wavelength discriminator 22 is configured with a plurality of layers configured to selectively transmit and reflect incident Stokes waves.
Given only as an example, wavelength discriminator 22 includes a multi-part layer 26 (blue) which is transparent to pump light 16. Accordingly, pump light 16 is coupled into Raman medium 18 without appreciative losses through input 20. The remaining pump light is decoupled from Raman medium 18 through another part of layer 26 coated upon pump light output 28 which is located immediately before output 24 for the desired Stokes.
The illustrated example is represented by Raman laser source 10 operative to output a first Stokes shown in yellow. Accordingly, all layers except for a layer 30, which defines output 24, reflect the desired first Stokes. The Raman phenomenon includes converting the energy of precedent Stoke or Stokes at higher frequencies to the desired subsequent stokes. In other words, as shown, the pump light at fundamental frequency transfers its energy to the desired first Stokes as soon as it reaches a threshold for the desired frequency. It may happen immediately upon the coupling of the pump light into Raman medium 18 or later upon first reflection of the pump light from wavelength discriminator 22. Furthermore, it is now the first Stokes that turns to be pump light for the second Stokes, which is undesirable in the current example. Accordingly, a layer 32 (blue) is coated upon a side 34 of Raman medium 18 from the very upstream end of the crystal so that whenever the second Stokes is generated and incident on side 34, it is sifted out right away. Meanwhile, red layer 38 and violet layer 40 are configured to reflect fundamental and desired (first) wave frequencies back into the Raman crystal. The reflected light at fundamental and desired first frequencies is incident on opposite side 36 of Raman medium 18 where layers 40, 38 and 32 are coated in the sequence opposite to that on side 32. Therefore waves at fundamental and desired frequencies are reflected back into the Raman crystal by respective layers 40 and 38, while outer layer 32 is being transparent to the second Stokes. The layers 40 and 32 extend over layer 26 which defines final output 28 for all undesired frequencies right before the desired Stokes wave exits the Raman crystal via output 24. The above disclosed operation of wavelength discriminator 22 continues along the entire length of the Raman crystal.
This undesirable effect is taken into account by the wavelength discriminator of
Referring to both
Referring to
Turning to
The Raman source 55 of
Referring to
The Raman laser source 55 further has an intermediate optical one-piece optical meniscus 62 which together with input coupler 60 define an inner cavity enclosing Raman medium 56. The intermediary meniscus 62 is configured with an inner face 64 partially reflecting and focusing the reflected desired (and lower frequency) Stokes wave(s) into Raman medium 56, while passing the rest of light further into the outer cavity. The transferred desired and parasitic Stokes waves are then fully decoupled through output coupler 58 while the pump light is fully reflected and focused into Raman medium 56 upon passing through intermediate meniscus 62. The presence of intermediate meniscus 62 allows the desired Stokes to traverse the inner cavity more times while the pump light completes a round trip through the outer cavity. Outside the outer cavity, the desired Stokes is further separated from the rest of decoupled light by any suitable means.
To design high efficiency longitudinal Raman converters several factors should be taken into account. For example, in the pulsed pumping schemes it is necessary to evaluate pump pulse delay, beam diameter for pump and Raman radiations, pump and Raman radiations overlap and focal position of pump and Raman radiations in the cavities.
Referring to
The dangerously high peak pump power within the Raman crystal can however be controlled in the scheme of
Referring to
As mentioned above, Raman mechanism is widely used in a great variety of applications. In fact, the frequency conversion problem is particularly important in the field of telescope optics. Atmospheric distortions require the use of guide stars to compensate for these distortions in telescopes. One form of guide star, in turn, is best produced by a laser source emitting at the sodium yellow line at 589 nm wavelength. Other forms of guide stars can be produced by illuminating the resonance transitions of alkali metals such as K, Li, Rb, or by illuminating other metal atom transitions for metals such as iron, or any metals formed in the upper atmosphere due to meteor bombardment. At present the 589 nm sodium line is most commonly used. The disclosed Raman laser source can be ideal for this application and have substantial advantages over other types of lasers. For example, typically, the 589 nm is conventionally derived by sum generation of two Nd:YAG lasers. One of these is a frequency-doubled Nd:YAG laser pumped by a dye laser. Such dye lasers require dye fluids which have limited lifetime and are subject to freezing or leaking. The first Nd:YAG emits at 1,064 nm and the second at 1.32 μm. By coincidence these two generate the sum of the desired 589 nm wavelength. However, it is well-known that the summing of two independent lasers requires careful control of the pulse timing such that the process remains efficient. In practice, pulse timing jitter prevents the stable generation of yellow output by sum generation of two Nd:YAG lasers.
Still another application that can benefit from the disclosed Raman laser source is an RGB engine. One of the possibilities in utilizing the disclosed Raman laser source is to use three operating in tandem with respective designated fiber laser pumps such as to produce all three colors. Still another possibility is to use a single Raman laser source as disclosed with multiple fiber laser pumps. And yet another possibility is to use a single fiber laser pump and single Raman laser source of this disclosure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US18/15906 | 1/30/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62452082 | Jan 2017 | US |
