Patent Grant
9042413
The present application relates to a maser emitting electromagnetic radiation at sub millimeter wavelengths, i.e., a smaser. More specifically, the application relates to a Q-switched solid state smaser operating at room temperature.
Solid state masers, where the maser gain medium is in solid state form are known. Typically, however, such solid state masers require operation in the ultra low temperature regime. Ruby has been used as a solid state gain medium for a solid state medium operating at about 60° K [C. R. Ditchfield and P. A. Forrester, ‘Maser action in the region of 60° K’ Phys. Rev. Letts., 1, p 448 (1958)].
More recently, organic masers have been demonstrated at room temperature by Oxborrow et al. [M. Oxborrow, J. D. Breeze and N. M. Alford, ‘Room-temperature solid-state maser’, Nature, 488, pp 353-356 (2012)]. The power output for such an organic maser was only −10 dBm at a masing frequency of 1.45 GHz.
According to one embodiment, there is provided a pulsed smaser, comprising: at least one optical resonator comprising: opposing mirrors; a solid state gain medium having a masing frequency in a range of from about 50 GHz to about 1 THz; and a Q-switch, wherein the solid state gain medium and the Q-switch are optically arranged between the opposing mirrors; and an optical pump arranged to provide optical pump power to the solid state gain medium, wherein the optical pump and the Q-switch are configured to generate pulsed masing in the solid state gain medium at the masing frequency at room temperature to provide output electromagnetic radiation at the masing frequency.
According to an aspect of the embodiment, the output electromagnetic radiation at the masing frequency has a peak power of about 1×107 Watts to about 1×109 Watts with a pulse width time duration of about 7 nanoseconds seconds.
According to another aspect of the embodiment, the solid state gain medium comprises at least one material selected from the group consisting of emerald, ruby, sapphire, titania, magnesium tungstate, zinc fluorite and yttrium oxide.
According to another aspect of the embodiment, the solid state gain medium is doped with at least one dopant ion from the group consisting of Gd, Cr, Ni, Fe, V and N.
According to another aspect of the embodiment, the optical pump comprises a plurality of lasers.
According to another aspect of the embodiment, the plurality of lasers comprise at least one of solid lasers, liquid lasers, gas lasers, laser-diodes, or light emitting diodes.
According to another aspect of the embodiment, the Q-switch is configured to provide Q-switching or mode-locking such that the output electromagnetic radiation has a pulse time in the pico-second to micro-second range.
According to another aspect of the embodiment, the Q-switch comprises a saturable-absorber material.
According to another aspect of the embodiment, the saturable-absorber material comprises at least one of graphene, graphane or carbon nano-tubes dispersed in a liquid-crystal.
According to another aspect of the embodiment, the output electromagnetic radiation is output in a TEM00 mode.
According to another aspect of the embodiment, the smaser further comprises a cooling element configured to cool the at least one solid state gain medium.
According to another aspect of the embodiment, the cooling element is configured to provide air cooling or liquid cooling.
According to another aspect of the embodiment, the cooling element comprises one or more heat dissipating fins.
According to another aspect of the embodiment, the at least one resonator comprises a plurality of resonators.
According to another aspect of the embodiment, an optical assembly comprises: the smaser; and an optical element arranged to receive output electromagnetic radiation from the smaser.
According to another aspect of the embodiment, the optical element comprises at least one of a lens or mirror arranged to expand and then focus the output electromagnetic radiation.
According to another aspect of the embodiment, the optical element comprises at least one of an afocal lens or an afocal mirror.
According to another aspect of the embodiment, the Q-switch comprises at least one of an acousto-optic Q switch, an electro-optic Q switch, a mechanical Q switch, or a passive Q-switch.
According to another embodiment, there is provided a continuous wave smaser, comprising: at least one optical resonator comprising: opposing mirrors; and a solid state gain medium having a lasing frequency in a range of from about 50 GHz to about 1 THz; and an optical pump arranged to provide optical pump power to the solid state gain medium, wherein the optical pump is configured to generate pulsed masing in the solid state gain medium at the masing frequency at room temperature to provide output electromagnetic radiation at the masing frequency
These and other features, aspects, and advantages of the present disclosure will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below, wherein like numerals denote like elements.
According to embodiments of the present invention, a smaser (a maser emitting electromagnetic radiation at sub millimeter wavelengths) is described which produces high peak power pulses at sub-mm wavelengths when operated at room temperature (300° K), A peak output of ˜1.3×108 Watts with a pulse width time duration around 7.5 nanoseconds is calculated for the smaser. The high power smaser is achieved by Q-switching, and including a solid state gain medium having a lasing frequency of from about 50 GHz to about 1 THz.
The optical resonator includes opposing mirrors 112a and 112b, solid state gain medium 114, and Q-switch 116. The solid state gain medium 114 and Q-switch 116 are optically arranged between the opposing mirrors 112a and 112b such that electromagnetic radiation reflecting between the mirrors 112a and 112b passes through the solid state gain medium 114 and Q-switch 116.
The solid state gain medium 114 has a lasing frequency of from about 50 GHz to about 1 THz. The solid state gain medium 114 may be, for example, emerald, ruby, sapphire, titania, magnesium tungstate, zinc fluorite or yttrium oxide. The solid state gain medium 114 may be doped. For example, the solid state gain medium 114 may be doped with at least one dopant ion where the dopant ion may be Gd, Cr, Ni, Fe, V or N, for example. The solid state gain medium 114 may have a geometry such that output electromagnetic radiation 118 from the smaser 100 is output in a TEM00 mode.
The Q-switch 116 may be configured to provide Q-switching or mode-locking such that the output electromagnetic radiation has a pulse time in the pico-second to micro-second range. Compared to mode-locking, Q-switching leads to much lower pulse repetition rates, much higher pulse energies, and much longer pulse durations. The Q-switch 116 may include a saturable-absorber material, for example. The saturable-absorber material may be at least one of graphene, graphane or carbon nano-tubes dispersed in a liquid-crystal, for example.
The optical pump 130 is arranged to provide optical pump power to the solid state gain medium. The optical pump 130 may comprise one or a plurality of lasers. The lasers of the optical pump 130 may be, for example, one or more solid lasers, liquid lasers, gas lasers, laser-diodes, or light emitting diodes. The optical pump 130 and the Q-switch 116 are configured to generate pulsed masing in the solid state gain medium 114 at the masing frequency at room temperature to provide the output electromagnetic radiation 118 at the masing frequency of the solid state gain medium 114.
The optical pump 130 may pump at radiation frequencies close to the emission frequency of the solid state gain medium 114 for efficiency purposes. For example, the optical pump 130 may pump at frequencies within a factor 2× or so of the main emission frequency of the solid state gain medium 114, if a suitable absorption line exists to do this, by beating optical lasers together inside the smaser cavity such that the beat-frequency is one that can be absorbed at energy levels just above the emission energy level.
The smaser 100 may further include a cooling element 140 configured to cool at least one solid state gain medium 114. The cooling element 140 may be configured to provide air cooling or liquid cooling, or example. The cooling element 140 comprises one or more heat dissipating fins 142. The cooling element 140 may comprise one or more fans. The cooling element may comprise a material having a high thermal conductivity contacting the solid state gain medium 114, such as sapphire, for example.
Q-Switch
A Q-switch is a device which can be quickly switched between states where it causes very low or rather high losses, respectively, for an electromagnetic radiation beam sent through the switch. Such Q-switch devices are typically used within a laser resonator with the purpose of active Q-switching the maser, where this is a technique for generating short intense pulses, where the pulse duration is typically in the nanosecond range.
The Q-switch 116 may be, for example, an acousto-optic Q-switch, such as an acousto-optic modulator, for example.
Alternatively, the Q-switch 116 may be an electro-optic Q-switch, where the polarization state of electromagnetic radiation can be modified via the electro-optic effect, a mechanical switch, or a passive Q-switch. Passive Q-switches are saturable absorbers which are triggered by the electromagnetic radiation itself passing through the saturable absorber. The losses introduced by the Q-switch in this case must be small enough to be overcome by the laser gain once sufficient energy is stored in the gain medium. The maser power then first rises relatively slowly, and once it reaches a certain level the absorber is saturated, so that the losses drop, the net gain increases, and the maser power can sharply rise to form a short pulse.
The selection of a Q-switch will depend upon the particular application. The following parameters may be considered, the operation wavelength, which influences e.g. the required anti-reflection coating, the open aperture, the losses in the high-loss state (particularly for high gain lasers), low-loss state (influencing the power efficiency), the switching speed (particularly for short pulse lasers), the damage threshold intensity, the required RF power, the cooling requirements, the size of the setup (particularly for compact lasers). Furthermore, the electronic driver must be selected to fit to the Q switch.
Mathematics for Q-Switching and Pulse Maser Power Output
The mathematics for Q-switching and the pulse maser are described below for calculating the pulse maser power output.
The maser material (gain medium) is characterized by the following parameters: No, the number of active ions in the volume element, τL, the lifetime of spontaneous (fluorescent) decay, and αo, the absorption coefficient of the unexcited maser material. The parameter αo is a function of the frequency, where the peak value at the center of the fluorescent line is taken as the parameter αo value.
The maser geometry is characterized by the following variables: V, the volume of the maser material, l, the length of the maser material, and L, the optical distance between the reflectors (mirrors) calculated while taking into account the refractive indices of the materials situated between reflectors.
The physical state of the maser is characterized by the following variables: φ, the photon density at the masing frequency ν, and N=N2−N1, the population inversion per unit volume, where N1 is the population per unit volume of the ground energy level and the N2 is the population per unit volume of the excited energy level.
An important device parameter is the loss coefficient γ, which is the fractional photon loss in a single passage between the reflectors. The loss coefficient γ may be subdivided as γ=γ1+γ2, where γ1 represents the fraction of photons emitted as useful output of the device and γ2 represents incidental losses.
The time of a single passage of a photon is t1=L/c, where c is the speed of light. Therefore, the lifetime of a photon within the Fabry-Perot interferometer of the maser cavity, i.e., the region between the cavity mirrors, is T=t1/γ. This is a fundamental unit of time characteristic of the maser.
The initial state for the formation of the pulse is achieved by pumping the maser with an optical source and keeping the loss coefficient at a value γ′ much higher than γ. During this period of excitation, the population inversion rises from −No to a positive value Ni, and the photon density also rises to a value φi, where the subscript i indicates that the values are “initial” values for the pulse. At time t=0 the loss coefficient is reduced to γ and the formation of the pulse begins.
Photons are amplified in the maser at the rate of φαx on traversing a distance x in the active gain material (gain medium). Here α, the coefficient of amplification, satisfies the equation α=αo N/No. The full length of the maser is traversed l/t1 times per second.
If np is the population inversion that corresponds to the threshold for the given maser, then αo lnp=γ.
At the start of the process for the formation of the pulse, the photon density φ is very low. It rises from φi, and reaches a peak φp, which is generally many orders of magnitude higher than φi. Then φ declines to zero because of the exhaustion of its source of energy supplied due to the population inversion. The population inversion is a monotonically decreasing function of time starting at ni and ending at nf.
The total energy obtainable from the pulse is proportional to ni-nf, and the peak power radiated is proportional to the peak photon density φp. The total energy and peak power are of prime interest; and may be calculated in terms of the initial values ni and φi. The maser itself is characterized by the parameters T and np, and further No and υ if a variation of materials is contemplated.
The total energy output of the pulse is given as:
E=½(nf−ni)VNohυ Equation 1
The peak power may be calculated noting that the peak is reached when n=np. After some manipulation the peak power, Wp, radiated from the maser is:
Wp=½[np log(np/ni)+ni−np](VNohυ/T) Equation 2,
where the photon-lifetime is T, defined above, and h is Planck's constant. For a reasonable example of Q-switching, taking ni=0.15 and np=0.05, it can be seen from
Further, the Q-switched pulse-width is well approximated by dividing the total pulse energy by the peak pulse power i.e., E/Wp.
Maser Parameter Design
In designing a maser for Q-switching for delivering a ˜mm wave coherent beam, the parameters for calculating the peak power, Wp from Equation 2 may be examined. As can be seen in
V may be made large by having a long cavity, while keeping it narrow to maintain lowest order TEM00 Gaussian-beam mode, which focusses to the smallest spot or beam-diameter.
Regarding No, in a molecular gas this might be ˜6 orders of magnitude less than in a solid, as shown as follows. In a molecular gas such as NH3 used for maser action at 23.86 GHz, at a pressure of say 300 milli-Torr, the gas equation PV=kNT, where P is pressure, T is now temperature in ° K, V is volume, k is Boltzmann's constant and N is the number-density of gas-molecules, may be used to calculate N, and estimate No. At 27° K, N=7.25×1013 molecules per cc.
Thus, looking at equation 2, it can be seen that changing the masing material from a gas to a solid state material to increase No, and increasing the frequency υ, can have a dramatic effect on peak output power.
Effect of Temperature and Operational Masing Frequency on Masing
The strength of the maser action in a maser crystal is proportional to the difference ni−nj between the populations of the two signal levels, the ground and excited energy levels, of the gain medium. The negative population difference at the signal frequency with the optical pump on is of the same order of magnitude as the population difference at thermal equilibrium with the pump off. The latter quantity is determined by the Boltzmann ratio exp(−hfij/kT), where fij is the frequency difference between the levels, i.e, the masing frequency. If the temperature is high, then hfij/kT<<1, and so the populations are nearly equal and the population difference will be small. With fij in the microwave range of a few GHz, at room temperature, then the population difference is very small, ˜1 in 104, and a substantial difference in population difference can only be achieved by cooling, typically to liquid-helium temperatures of ˜4° K
The situation for a greater masing frequency with a masing wavelength in the mm range is different. Table 1 illustrates the Boltzmann population ratio (exp term) and population difference for temperatures of 4° K and 300° K (room temperature) for different masing frequencies.
As can be seen in Table 1, by raising the operating frequency (masing frequency) well above the normal maser operating frequencies of a few GHz to some 10 s of GHz, when a frequency of 200 GHz to 240 GHz is reached, the same population ratios and differences are reached at room temperature that require 4° K temperatures for the lower microwave frequency of 3 GHz. Thus, cooling of the gain medium is not needed at 200 GHz to 240 GHz frequencies to achieve similar population inversion values as that achieved at 3 GHz at 4° K.
Moreover, the high frequency lasing greatly increases the peak output power based on the Q-switching as can be from Equation 2.
Further by employing a solid state gain medium in place of a gas, the peak output power also greatly increases as can be seen from Equation 2, as No, the number of active ions in the volume element, is many orders-of magnitude greater in solids than gases.
Maser Materials for 50 GHz to 1 THz Operation
Solid State gain medium materials in the 50 GHz to 1 THz are now contemplated. Based on calculations of the masing frequency, the following materials are as follows: Yttrium Oxide, Y2O3, with Cr3+ ions, operating at 72.7 GHz, Zinc Fluorite, ZnF2, with Ni2+ ions, operating at 80, 86 or 166 GHz, Magnesium Tungstate, with Fe3+ ions, operating at 137.2 GHz, Sapphire, Al2O3, with V3+ ions, operating at 210 to 230 GHz, Titania, TiO2 with N2+ ions, operating at 124.6 (Fe3+ ions) or 250 GHz.
Here it is assumed that at least ˜3*1019 ion spins per cc is possible in the material chosen for the gain medium.
Peak Output Power for Sample Q-Switched Smaser System
A smaser system is now described where the peak output power is calculated.
The smaser cavity is of circular cross-section. In this case, Kogelnik-Li equations [H. Kogelnik and T. Li, ‘Laser beams and resonators’, Proc. IEEE, 54, p 1312 (1966).] may be used for the TEM00 mode, as set out with respect to
The Kogelnik-Li equations provide:
Beam radius wz at distance z is
Radius of curvature of wavefront Rz is
Axial position of waist wl
Radius of waist wl
from which the beam waist may be calculated.
Further, the system employs optical-pumping of a 3-level smaser system in the solid state gain medium. Such pumping may be accomplished using near infrared laser diodes, for example.
The operating frequency (masing frequency) of the system is about 250 GHz, and thus Sapphire, Al2O3, with V3+ ions or Titania, TiO2 with N2+ ions, may be chosen as the gain medium, where the gain medium length is 30 cm and the gain medium diameter is 2.5 cm. Further, a value of 25×1019 ions per cc for the gain medium is presumed for the system. In this case, a spontaneous lifetime of the gain material is calculated to be ˜3×105 seconds and the stimulated photon lifetime to be ˜2×10−9 seconds, via the equation: spontaneous lifetime=(3·h·c3)/(64·π4·υ3·μ2), where h is Planck's constant, c is the speed of light, υ is the frequency of smaser emission, and μ is the electric dipole moment. The stimulated lifetime is approximately the inverse linewidth of the emitted spectrum of the smaser.
The loss per cm of the gain material in the cavity may be calculated following Troup's scaling-equation for frequency, QL=2πf0W/(Total power lost), where QL is the total load Q of the smaser cavity, fo is the smaser emission frequency, and W is the peak power [G. Troup, Masers, Methuen, (1959)]. This provides a value of 0.01 per cm loss.
The gain per cm may be calculated based on Yariv's approach [A. Yariv, Quantum Electronics, Wiley, (1989), 3rd Edition, page 170], where the gain coefficient γ is provided by:
where the atom density is N2 (atoms/m3) in level 2 and N1 (atoms/m3) in level 1. The factor g2/g1 accounts for the degeneracy g2 of level 1. λ is the wavelength, η is the transition efficiency≈1, n is the refractive index of the material, tspont is the spontaneous lifetime, and g(υ) the linewidth. This provides a value of ˜2 per cm. To be conservative, the actual gain per cm is considered to be smaller by a factor of ten, that is, it is ˜0.2 per cm.
The mirror reflectivities of the opposing mirrors are taken to be 0.99 and 0.9, respectively, for the mirror receiving the input pump beam, and the mirror outputting the output beam, where the mirrors at positioned at opposing ends of the cylindrical cross-section maser cavity. The smaser cavity is end pumped using optical radiation.
Assuming the Q-switching population parameters discussed earlier, ni/np=3, in
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention.
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