Electrodeless lighted laser-driven light source

Abstract

An electrodeless laser-driven light source includes a laser that generates CW continuous light. A pump laser generates pump light. A Q-switched laser crystal receives the pump light generated by the pump laser and generates pulsed laser light at an output in response to the generated pump light. A first optical element projects the pulsed laser light along a first axis to a breakdown region in a gas-filled bulb that includes an ionized gas. A second optical element projects the CW continuous light along a second axis to a CW plasma region in the gas-filled bulb that includes the ionized gas. A detector detects plasma light generated by the CW plasma and generates a detection signal. A controller generates a control signal that controls the pump light to the Q-switched laser crystal so as to extinguish the pulsed laser light for a delay time after the detection signal exceeds a threshold.

Description

[0001] The section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described in this application in any way. Technical Field Background Technology

[0002] Many commercial and academic applications require high-brightness light over a wide wavelength range. For example, laser-driven light sources are available that offer high brightness, reliability, and long lifespan across a spectral range from extreme ultraviolet to visible and infrared light. Several such high-brightness light sources are manufactured by Energetiq, a Hamamatsu Company, located in Wilmington, MA.

[0003] The demand for high-brightness light sources is growing across a wide range of fields, including biology, chemistry, climate, and physics. These applications include, for example, semiconductor metrology, sensor calibration and testing, shaped light generation, surface metrology, spectroscopy, and other optical measurement applications. Therefore, advancements are needed in such high-brightness light sources that improve the size, cost, complexity, reliability, stability, and efficiency of this important type of broadband light source. Summary of the Invention Attached Figure Description

[0004] The present teachings and their further advantages are described in detail below with reference to the accompanying drawings, based on preferred and exemplary embodiments. Those skilled in the art will understand that the drawings described below are for reference only. The drawings are not necessarily drawn to scale; the focus is generally on illustrating the principles of the teachings. The drawings are not intended to limit the scope of the applicant's teachings in any way.

[0005] Figure 1A An embodiment of an electrodeless laser-driven light source according to the present teachings is shown, which uses a pulsed laser projected into an inflatable bulb along one axis and a CW laser projected into an inflatable bulb along a different axis.

[0006] Figure 1B An embodiment of an electrodeless laser-driven light source according to the present teachings is shown, which uses a pulsed laser projected into an inflatable bulb along one axis and a CW laser projected into the inflatable bulb along the same axis.

[0007] Figure 2A An image of a gas-filled bulb in an embodiment of an electrodeless laser-driven light source according to the present teachings is shown, illustrating emission using only pulsed laser excitation.

[0008] Figure 2B Show Figure 2AThe image shown is of an inflatable bulb, illustrating emission only under CW laser excitation.

[0009] Figure 3 A flowchart illustrating the steps of a method for illuminating plasma in an electrodeless laser-driven light source according to the teachings herein is shown.

[0010] Figure 4 A set of oscilloscope traces are shown for pulsed laser illumination, pumped laser illumination, and plasma emission of an electrodeless laser-driven light source according to the teachings herein.

[0011] Figure 5A An embodiment of a Q-switched crystal including a gain region and a saturable absorber region, according to the teachings, is shown.

[0012] Figure 5B An embodiment of a YAG-based passive Q-switched laser bar according to the present teachings is shown, the laser bar having a curved surface suitable for an electrodeless laser driving source.

[0013] Figure 6 A graph showing the pulse energy and pump current threshold of a pump laser as a function of the pulse length of a quasi-CW pump pulse, which generates a laser pulse sufficient to cause gas breakdown, is shown. This quasi-CW pump pulse is used in an embodiment of an electrodeless laser-driven light source according to the present teachings.

[0014] Figure 7 A bare bulb with a focusing lens assembly is shown in an embodiment of an electrodeless laser-driven light source according to the present teachings. Detailed Implementation

[0015] The present teachings will now be described in more detail with reference to exemplary embodiments illustrated in the accompanying drawings. While the present teachings are described in conjunction with various embodiments and examples, they are not intended to be limited to these embodiments. Rather, as will be understood by those skilled in the art, the present teachings include various alternatives, modifications, and equivalents. Those skilled in the art, upon understanding the teachings herein, will recognize other implementations, modifications, and embodiments, as well as other areas of use, within the scope of this disclosure.

[0016] The term "implementation" or "one implementation" as used in this specification means that a particular feature, structure, or characteristic described in connection with an implementation is included in at least one embodiment of the teachings. The phrase "in one implementation" appearing in different places in the specification does not necessarily refer to the same implementation.

[0017] It should be understood that, as long as this teaching remains operable, the steps of the methods described herein can be performed in any order and / or simultaneously. Furthermore, it should be understood that, as long as this teaching remains operable, the apparatus and methods described herein can include any number or all of the aforementioned embodiments.

[0018] Laser-driven light sources use CW lasers to directly heat gaseous plasma to the high temperatures required to generate broadband visible light. High-brightness laser-driven light sources offer significant advantages over light sources that use high-voltage electrodes to sustain the plasma. Laser-driven light sources rely on optical discharge plasma, rather than the electrical discharge plasma used in devices such as arc lamps. In electrical discharge lamps, the electrode material can evaporate, altering the characteristics of the discharge over the lamp's lifespan. This shortens the lamp's lifespan. Furthermore, electrode-based systems result in thermal, mechanical, and electrical stresses on the light source. Known laser-driven light sources do not rely on electrodes to sustain the plasma, but they still use electrodes to illuminate the plasma.

[0019] Known electrode-based light sources can have significant limitations. For example, electrode-based light sources have limitations in lamp holder size and bulb mounting methods. Electrode-based light sources must be designed to avoid parasitic arcing and require sufficient volume in the lamp holder for the electrodes and lighting circuitry. Electrode-based light sources limit the cold-fill pressure of the bulb; for example, the glass-metal seal of the electrodes can limit the maximum fill pressure. Furthermore, the larger bulb size of electrode-based light sources can also affect the bulb fill pressure. Electrode-based light sources also have limited bulb shapes they can accommodate. This is because, for example, electrode-based light sources require the positioning, fixing, and connection of electrodes. These design limitations can lead to noise within the light source.

[0020] Therefore, providing electrodeless laser-driven light sources can improve reliability and performance, reduce cost and complexity, and offer other benefits. Illuminating plasma using optical illumination requires careful design and control of the light source and associated light transmission mechanisms. A feature of this teaching is the provision of electrodeless laser-driven light sources. In these sources, the plasma is illuminated by optical illumination, rather than by electrical energy supplied by electrodes, as is the case in known high-brightness laser-driven light sources.

[0021] Electrodeless laser-driven light sources offer numerous advantages and benefits. Compared to existing light sources, electrodeless sources can be implemented using smaller bulbs and have higher maximum fill pressures. Higher fill pressures result in higher brightness, especially within certain laser power ranges. Electrodeless sources are free from contamination by electrode materials. Furthermore, there are fewer geometric constraints on the lamp tube shape. Typically, a smaller lamp base can be used for the same characteristics. Additionally, the absence of active electrical components for power supply reduces the need for associated power supplies, control electronics, and electrical connections, significantly decreasing the number of required components. However, some implementations of electrodeless laser-driven light sources can be achieved within existing lamp tube packages that are laser-driven light sources with electrodes. This is at least partly because electrodeless devices are less complex and smaller in size compared to electrode-driven laser light sources.

[0022] Figure 1A This illustration shows one embodiment of an electrodeless laser-driven light source 100 according to the present teachings, which uses a pulsed laser 102 projected along one axis 106 into a gas-filled bulb 104 and a CW laser 108 projected along a different axis 110 into the gas-filled bulb 104. The pulsed laser 102 is generated using a Q-switched crystal 112, which is pumped by a pump laser 114 generated by a pump laser 116. A coupling optics element 118 is used to couple the pump laser 114 to the Q-switched crystal 112. An optics element 120 is used to direct the pulsed laser 102 generated in the Q-switched crystal 112 toward the bulb 104 and to direct the pump laser 114 away from the bulb. In some embodiments, the optics element 120 is a dichroic optics element. The energy of the pulsed laser 102, provided to the plasma breakdown region 126, illuminates the plasma.

[0023] A CW laser generates a CW laser 108. An optical element 124 projects and / or focuses the continuous CW light onto a region of bulb 104 containing a plasma breakdown region 126. In some embodiments, the optical element 124 is a focusing element, such as a lens. An optical element 128 projects and / or focuses a pulsed laser 102 onto a region of bulb 104 containing a plasma breakdown region 126. In some embodiments, the optical element 128 is a focusing element, such as a lens. The region illuminated by the pulsed laser is referred to as the pulsed illumination region and has a well-defined location and shape based on a projection element for directing visible light from the Q-switched crystal 112. Energy supplied from the pulsed laser 102 to the plasma breakdown region 126 illuminates the plasma. The region illuminated by the continuous CW light is referred to as the continuous CW illumination region and has a well-defined location and shape based on a projection element for directing visible light from the CW laser 122. Energy supplied from the CW laser 108 to the plasma breakdown region 126 sustains the plasma.

[0024] Plasma breakdown region 126 generates CW plasma light 130. CW plasma light 130 is incident on detector 132. CW plasma light 130 can be directed to detector 132 via optical element 120 or free space and / or other optical transmission methods. Detector 132 generates a detection signal at an output connected to controller 134. Controller 134 is connected to the control input of pump laser 116. Controller 134 generates a control signal that controls the parameters of pump light 114 directed to Q-switched laser crystal 112. In some embodiments, controller 134 is configured to control the parameters of pump laser 116 and pump light 114 to extinguish pulsed laser 102 within a delay time after the detection signal from detector 132 exceeds a predetermined threshold level.

[0025] A high peak power pulsed light 102 is required to illuminate the plasma in the plasma breakdown region 126. However, to generate a sustained CW plasma light 130, the pulsed light must not exceed a certain threshold, which can occur if a pulse is present in the plasma after a predetermined delay following the plasma light reaching the predetermined threshold. Excessive pulsed light can extinguish the plasma. By extinguishing the pulsed light before reaching the threshold, the plasma light can be sustained by applying only the sustained CW light 108.

[0026] In some implementations, a certain number (e.g., one or more) of pulses in the pulsed light are required to illuminate the plasma, but after illumination, additional pulses quench the plasma. Therefore, once the plasma light 130 is detected at detector 132, the pulse is extinguished after the illumination pulse and before the next pulse is generated. It should be understood that this configuration is merely one example of this aspect of the teachings. Various algorithms and thresholds can be used by systems according to the teachings to extinguish the pump light 114 after various delays relative to the generation of the plasma light 130. These parameters depend on various factors, such as the type, density, and / or temperature of the gas. Furthermore, these parameters also depend on the relative power of the pulsed light and the CW continuous light 108. Additionally, these parameters depend on the focusing and other optical characteristics of the optics 124, 128. Moreover, these parameters also depend on the energy density of the CW continuous light 108 and / or the pulsed light 102 in the plasma breakdown region.

[0027] A key feature of this teaching is that the axes of the pulsed light and the CW continuous light can be positioned in various relative ways. For example, refer to... Figure 1A The axes 106 and 110 are nominally orthogonal. Alternatively, an electrodeless laser drive source can be configured so that the pulsed light and CW light are on the same axis.

[0028] Figure 1BAn embodiment of an electrodeless laser-driven light source 150 according to the present teachings is shown, which uses a pulsed laser 152 projected along one axis into an inflatable bulb 154, and a CW laser 156 projected along the same axis into the inflatable bulb. The electrodeless laser-driven light source 150 and... Figure 1A The electrodeless laser-driven light source 100 in this example has many common features. A pump laser 160 generated by a pump laser 162 pumps a Q-switched crystal 158. A coupling optics element 164 (in some embodiments, a collimating package that nominally collimates the light) receives the light generated by the pump laser 162 and directs it toward the Q-switched crystal 158.

[0029] Optical element 166 is used to direct the pulsed laser 152 generated in the Q-switched crystal 158 toward the bulb 154. Optical element 166 can also direct the pump laser 160 away from the bulb 154 (e.g., by reflection). In some embodiments, optical elements 166 and 168 are dichroic optical elements. In some embodiments, optical element 166 can transmit light 152 at a pulsed wavelength or CW laser 156. CW laser 170 generates CW laser 156. Optical element 172 projects and / or focuses the CW continuous light 156 and the pulsed laser 152 onto a region of the bulb 154 containing the plasma breakdown region 174. In some embodiments, optical element 172 is a focusing element, such as a lens.

[0030] Plasma breakdown region 174 generates CW plasma light 176. CW plasma light 176 is incident on detector 178. CW plasma light 176 can be directed to detector 178 via optical element 166 or free space and / or other optical transmission methods. Detector 178 generates a detection signal at its output in response to the detected CW plasma light. The output of detector 178 is connected to controller 180. Controller 180 is connected to the control input of pump laser 162. Controller 180 generates a control signal that controls the parameters of pump light 160 directed to Q-switched laser crystal 158. In some embodiments, controller 180 is configured to control the relevant parameters of pump laser 162 and pump laser 160 to extinguish pulsed laser 152 within a delay time after the detection signal from detector 178 exceeds a predetermined threshold level. In some embodiments, collimation package 182 is used to collimate the light from CW laser 170. Optical element 184 (which may include one or more optical elements) is used to focus pump laser 160 onto Q-switched crystal 158.

[0031] Another feature of this teaching is that, in some embodiments, pulsed light and continuous CW light can be oriented to form independent illumination regions within the plasma region. These two illumination regions can be distinct or partially or completely overlap, depending on requirements. Control of the relative position and shape of the pulsed illumination region and the continuous CW illumination region can be used to provide a specific spatial distribution of energy delivered by these two light sources. As further described herein, the energy density of each of the continuous CW light and the pulsed light affects the illumination and persistence of the plasma. Therefore, the ability to control the relative position and shape of the pulsed illumination region and the continuous CW illumination region allows for control of the energy density distribution supplied to the plasma. This arrangement affects the ability of the illuminated plasma generated in the pulsed illumination region to transition to the CW illumination region as a stable CW plasma.

[0032] Figure 2A Image 200 of a gas-filled bulb in an embodiment of an electrodeless laser-driven light source according to this teaching shows emission under pulsed laser excitation only. Images 202 and 204 are both shown as side views, but offset at an angle, to show the three-dimensional positioning of the pulsed illumination region and the CW laser focus. The extent and location of the pulsed illumination region are visible in these images 202, 204. Note that the location of the pulsed breakdown depends on the pulse energy. As the energy increases, the breakdown location shifts toward the pulsed laser. The three-dimensional alignment of the pulsed laser-broken plasma with respect to the pulsed light and with respect to the CW laser focus enables the method to operate at lower CW laser power and / or lower pulse energy.

[0033] Figure 2B Show Figure 2A Image 250 shows the inflatable bulb, illustrating emission using only CW laser excitation. Images 252 and 254 are both shown as side views, but offset at an angle, to show the three-dimensional localization of the pulsed illumination region and the CW laser focus. The extent and location of the pulsed illumination region are visible in these images 252 and 254. Figure 2A Images 202 and 204 also show the outlines 256 and 258 of the pulsed illumination region. In this embodiment, the relative positions and shapes of the pulsed illumination region and the CW continuous illumination region are such that the two regions are distinct and do not overlap. The pulsed illumination region is smaller because the light is more tightly focused, resulting in a higher pulse energy density delivered to the plasma.

[0034] Figure 2AThe -B images were collected during the experiment to determine the operating parameters of the pulses providing plasma illumination. Some details of the experimental conditions are described below. For example, stable plasma illumination in a xenon-filled bulb can be achieved in the energy range of 135 to 225 microjoules with pulses of 2 kHz pulse frequency and 1-ns duration. The wavelength of the pulsed light is 1064 nm. For this specific example of the experimental configuration, the threshold energy of the breakdown light is 135 microjoules to achieve stable CW plasma. Furthermore, at 210 microjoules, the plasma illuminates and can eventually stabilize, but there can be on-and-off cycles of the CW plasma before stable operation. Illumination can also be achieved at 225 microjoules. Above 240 microjoules, the CW plasma is not stable in some configurations. The relative positions of the pulsed light illumination and the CW light illumination are crucial. Adjusting the alignment of the bulb and the CW laser along the optical axis of the pulsed laser can improve or “turn off” illumination. After illuminating the CW plasma, the CW laser power can be reduced to 8-10 watts, but the CW laser plasma can still be maintained. Reliable pulsed-to-CW transition is possible at any CW laser power value above 15.5 watts. There is no upper limit to CW laser power. As those skilled in the art will understand, beam quality affects the energy delivered to the gas at a given laser power.

[0035] Another feature of the electrodeless illumination method in this teaching is the recognition that CW plasma illumination can be achieved as a two-step process. In the first step, xenon gas is broken down by a laser pulse. The second step involves transitioning or "switching" from pulsed plasma to continuous CW plasma. Then, after continuous plasma has begun to be generated and the switching has been successful, the pulsed laser is turned off. For example, the switching can be achieved by turning it off during the "first beam" of the CW plasma. It is important to turn off the pulsed light after illumination before other pulses, as subsequent pulses can break down the CW plasma light. The number of pulses that can be tolerated depends on the power of the CW plasma light. In some implementations, the delay time for pulse extinguishing is short enough that the next pulse can extinguish within that time after the illumination pulse.

[0036] Figure 3A flowchart 300 illustrates the steps of a method for illuminating plasma in an electrodeless laser-driven light source according to this teaching. Flowchart 300 shows steps for controlling the delivery of energy to a plasma region contained within a gas-filled bulb to generate high-brightness light, wherein the plasma is illuminated by illumination rather than by electrical energy supplied by electrodes, as in prior art high-brightness light sources. In a first step 302, electromagnetic energy is supplied to the gas within the bulb using a continuous CW laser. The CW laser can be referred to as a CW continuous light. In some embodiments, the gas is xenon. In a second step 304, a laser pulse is generated by supplying a pump laser to a Q-switched crystal. A feature of this teaching is the recognition that the pulse supplied by the Q-switched crystal can be controlled by controlling the application of pump light to the crystal. In a third step 306, the laser pulse is supplied to the gas within the bulb, which creates a breakdown region in the gas. In a fourth step 308, the CW laser supplied to the bulb is absorbed when the ions and electrons generated by the pulsed light reach an appropriate density. In step 310, the absorbed CW laser excites ions and electrons, generating CW plasma, which emits high-brightness light in the plasma region. That is, the electromagnetic energy provided by the continuous CW light causes the bulb to generate CW plasma light.

[0037] In step 6, 312, a portion of the high-brightness light emitted by the bulb is detected in the detector. The detector, in response to the received portion of the CW plasma light, generates a plasma detection signal. This signal can be provided to the controller. In step 7, 314, the pump laser stops when the plasma detection signal exceeds a predetermined threshold. Pumping stops, causing pulse interruption. That is, the pulse light extinguishes in response to the pump light stopping and reaching the Q-switched crystal. In some embodiments, the controller stops the laser pump. In step 8, 316, the CW plasma is maintained using a CW continuous light source. We note that in some embodiments, the CW continuous light source is referred to as a continuous light source generated by operating a pulsed laser at a high pulse rate.

[0038] The various embodiments of the electrodeless laser-driven high-brightness light source described in this teaching utilize different parameters of the light supplied to the gas. For example, the repetition rate of the Q-switched laser pulses can be controlled. The pulse energy of the pulsed light supplied to the gas can be controlled. The duration of the Q-switched laser pulses can also be controlled. Furthermore, the power of the CW laser can be controlled. In some embodiments, the pulse repetition rate of the pulsed laser is between 1 kHz and 20 kHz.

[0039] Experimental and / or theoretical evaluations have determined, for example, that high-quality CW plasma can be provided when the Q-switched laser crystal is configured such that the pulse repetition rate of the pulsed laser is less than or equal to 1 kHz. Continuous-wave plasma can be generated when the Q-switched laser crystal is configured such that the pulse energy of the pulsed laser is in the range of 50 to 500 microjoules.

[0040] Continuous wave plasma is generated under various pulse energies, pulse durations, and CW power conditions depending on a specific configuration. For example, continuous wave plasma is generated when the Q-switched laser crystal is configured such that the pulse energy of the pulsed laser is between 500 microjoules and 5 millijoules. Furthermore, continuous wave plasma can be generated when the Q-switched laser crystal is configured such that the pulse duration of the pulsed laser is between 0.1 ns and 10 ns. Continuous wave plasma can also be generated when the CW laser source is configured such that the power of the CW continuous beam is in the range of 5 W to 50 W. Additionally, continuous wave plasma can be generated when the CW laser source is configured such that the power of the CW continuous beam is in the range of 5 W to 1500 W. The above ranges are merely examples of operating ranges and do not limit the teachings in any way.

[0041] A key feature of this teaching is that the detection of CW plasma can be used to control the illumination of the pulsed laser. In some embodiments, this control can prevent the extinguishing of the CW plasma light pulses or other adverse effects after the plasma has been illuminated. Figure 4 A set of oscilloscope tracks 400 is shown according to this teaching, for pulsed laser illumination, pumped laser illumination, and plasma emission from an electrodeless laser-driven light source. This set of oscilloscope tracks 400 illustrates the timing of laser operation and plasma light generation in an embodiment of the electrodeless laser-driven light source of this teaching. Figure 4 In the diagram, the measured pulsed laser illumination trajectory 402, the measured pump laser illumination trajectory 404, and the measured plasma emission trajectory 406 are shown as a time function in a set of oscilloscope trajectories 400. The presence of pump light shown in trajectory 404 produces two pulses 408 and 410 visible in pulsed light trajectory 402. Plasma illumination begins after the second pulse 410, resulting in an increase in the measured CW plasma light shown in trajectory 406.

[0042] The controller used in the system that generates this data is configured such that the pump is turned off 414 when the detected CW plasma light, shown in trajectory 406, reaches threshold 412. The quenching pulse from the Q-switched crystal is shown in trajectory 402. (Instead of or except) Figure 4The conditions shown in the set of oscilloscope traces 400 can be configured in various ways. The pump laser can be switched off or extinguished immediately after a threshold for CW plasma light is achieved. Alternatively, the pump laser can be switched off or extinguished after a predetermined delay after a threshold for CW plasma light is achieved. In various methods according to this teaching, various thresholds can be used to achieve the desired performance.

[0043] In some embodiments, the detection signal represents the power of the plasma light, and the threshold is selected as the desired ratio of the plasma light power to the operating power. In some embodiments, the desired ratio is typically 50%. In other embodiments, the desired ratio is typically 90%. In still other embodiments, the desired ratio is in the range of 30% to 95%.

[0044] A predetermined delay that has a specific relationship to the pulse period of the pulsed laser can be selected. This allows the pump to be turned off before the next pulse in the pulse sequence is generated. In some embodiments, the period between pulses in the pulsed laser is greater than the delay time. In some embodiments, the controller is configured such that the delay time is less than one pulse period of the pulsed laser. It should be understood that the pulse period can be controlled by adjusting the Q-switched crystal and / or the pump configuration and power level.

[0045] A feature of this teaching is that different known Q-switching crystals can be used. The wavelength of the pulsed light should be suitable for inducing breakdown of one or more gases in the bulb. Figure 5A An embodiment of a Q-switched crystal 500 according to the present teachings is shown, the crystal including a gain region 502 and a saturable absorber region 504. As those skilled in the art will understand, different host materials and dopants can be used to provide suitable gain regions 502 and saturable absorber regions 504. For example, the host material of the crystal 500 may be a glass host, a yttrium aluminum garnet host, or a spinel host. For example, ytterbium dopants, chromium dopants, cobalt dopants, or vanadium dopants may be incorporated into one or both regions of the gain region 502 and the saturable absorber region 504 of the crystal 500. The Q-switched crystal may also include a narrowband filter, for example, for reflecting at least a portion of the plasma light and / or blocking wavelengths in the xenon spectrum. In some embodiments, the Q-switched laser crystal has a coating on one surface. For example, this coating may be a protective coating, a reflective coating, and / or an anti-reflective coating.

[0046] Figure 5BAn embodiment of a yttrium aluminum gallium (YAG)-based passive Q-switched laser rod 550 is shown, having a curved surface 552 suitable for an electrodeless laser-driven source according to the present teachings. A saturable absorber region 554 is a chromium dopant in a yttrium aluminum garnet body. A gain region 556 is a ytterbium dopant in a yttrium aluminum garnet body. The dopants and the body help set the wavelength of the pulsed light and the rise and fall times of the pulse. The lengths of the saturable absorber regions 558, L2, the gain regions 560, L1, and the crystal width 562, W are selected to provide desired output pulse parameters, including pulse repetition rate, pulse duration, pulse energy, etc.

[0047] Q-switched crystals are a mature technology. For example, Q-switched crystals are used in known passive Q-switched microchip lasers. To give a specific example, a microchip laser uses... Figure 5B Similar to the crystal 550 described herein, this microchip laser has a saturable absorber region length of 558, L2 = 1.36 mm, a gain region length of 560, L1 = 3 mm, and a crystal width of 562, W = 3 mm. This microchip laser is capable of providing 1.6 ns pulses with 74 microjoules of energy at a repetition rate of 14 kHz, achieved with a pump power of 10 W on a 970 nm pump laser. With increasing pump power, the average output power and the generated pulse repetition rate can be increased to 1 W and 13.6 kHz, respectively, at a pump power of 9.3 W. Maximum output power is reached without observing thermal flipping. The average pulse width is 1.58 ± 0.04 ns. In practice, the achieved pulse energy and peak power are 73.8 ± 0.7 μJ and 46.0 ± 0.8 kW, respectively. One feature of this teaching is that stepless illumination can be achieved using pulsed light parameters, which can be achieved by these highly available, compact, and reliable optical pulse sources provided by Q-switched crystals 500 and 550.

[0048] Pump efficiency and pulse output depend on various characteristics of crystals 500 and 550. For gain crystals 502 and 556, these characteristics include the dopant element (e.g., YB or Nd), doping percentage, and diameter and length. For saturable absorber crystals 504 and 552, these are the dopant element (e.g., chromium or vanadium), doping percentage, initial absorption percentage, diameter, and / or length. In some embodiments, reflective and / or transmissive coatings for the pump wavelength and pulse wavelength are provided on one or more ends of crystals 500 and 550. For example, a Yb:YAG-Cr:YAG bonded crystal may include a coating on the Yb:YAG end that has high transmittance at a wavelength of 940 nm and high reflectivity at a wavelength of 1030 nm. Additionally, on the Cr:YAG end, the crystal coating only partially reflects at a wavelength of 1030 nm (i.e., the output coupler). While many Q-switched lasers are pumped with the saturable absorber and output coupler located at the end opposite the input pump laser, the output coupler for electrode-free pulsed Q-switched crystals can be located at the pump input end, rather than at the saturable absorber end.

[0049] Some implementations of crystals 500 and 550 may have undoped ends around the Yb&CrYAG, which can be called non-absorbing mirrors. This configuration avoids thermal overload and facet failure. In the gain regions 502 and 556, Nd:YAG gain media are common and relatively inexpensive. Nd:YAG gain regions 502 and 556 are pumped at 808 nm and emit light at 1064 nm. Yb:YAG gain regions 502 and 556 are less common and more expensive. This material is pumped at 940 or 970 nm wavelengths and emits light at 1030 nm. These Yb:YAG crystals are typically coated to accommodate 940 nm pumping. Crystals coated for 940 nm will not work efficiently at 970 nm (e.g., a crystal coated at 940 nm has only 60% transmittance at 970 nm). Furthermore, light at a wavelength of 940 nm is generally easier to separate from light at 1030 nm than light at 970 nm. Ytterbium-doped glass can also be pumped at 975 nm. This pump wavelength is the same as the laser wavelength of known laser-driven light sources.

[0050] When designing a Q-switched crystal for generating electrodeless bright pulses according to this teaching, some important characteristics include the choice of laser wavelength, the order of coatings, the gain portion, the saturable absorber portion, and the directions of the pump pulse input and output. Other important characteristics include combining / separating the pump beam and the plasma beam, and the need to protect the CW laser from the pulses generated by the Q-switched crystal. (See again...) Figure 1A-B. Different implementations of the light sources 100 and 150 have different configurations for the positions of the pump lasers 116 and 162, the Q-switched crystals 112 and 158, and the CW lasers 122 and 170, and these configurations influence these design choices. Furthermore, due to the high pulse energy, crystal mounting and related thermal management are also important considerations.

[0051] Figure 6 A graph showing the pulse energy and pump current threshold of a pump laser as a function of the pulse length of a quasi-CW (QCW) pump pulse, which generates a laser pulse sufficient to cause gas breakdown, is used in embodiments of electrodeless laser-driven light sources according to this teaching. That is, the pulse length is the width of a repeating pulse (e.g., the width of a pulse in a square wave signal) used to generate the quasi-CW pump light signal. Figure 600 shows measurements taken with a xenon-containing bulb. Figure 600 shows an example operating point and illustrates operation over a range of pulse durations. The threshold tends to plateau when the pulse length exceeds approximately 500 microseconds. It should be noted that the operating parameters of various embodiments of the light source according to this teaching may differ from those shown in the example data. As specific examples, some examples of operating parameters for pulsed lighting and transition switching for a xenon-filled cold light bulb at 22 atmospheres are as follows: (1) CW transition switching can be achieved at 980nm with a CW laser power as low as 14W; (2) CW transition switching can be achieved with a CW laser center wavelength as low as 972nm; (3) near-instantaneous CW transition switching can be achieved with a CW laser center wavelength as low as 975nm; and (4) CW transition switching can be achieved with a CW laser power up to 50W. When the laser spectrum content is zero at a wavelength of 980nm, the transition switching can take several seconds to one or two minutes. When the CW laser power is 20W, the center wavelength changes by 1-2nm relative to the 980nm center wavelength for a successful transition. In the case of a cold light bulb at 30 atmospheres, CW transition switching can be achieved at a CW laser power of 30W and a center wavelength of 976nm. Generally, lighting is more robust with higher pressure bulbs compared to lower pressure bulbs. For example, typically, light bulbs with pressures exceeding 30 atm provide more robust illumination compared to bulbs with pressures of approximately 22 atm.

[0052] Figure 7A bulb system 700 used in one embodiment of an electrodeless laser-driven light source according to the present teachings is shown, comprising a bare bulb 702 and focusing lens assemblies 704, 706. A plasma region 708 is shown. Focusing lens assemblies 704 and 706 are configured in planes at a 90-degree angle to each other. One assembly 704 directs pulsed light to the plasma region 708 in the bulb 702, while the other assembly 706 directs continuous CW light to the plasma region 708 in the bulb 702. As described herein, the shapes of the pulsed illumination and the continuous CW illumination in the plasma region 708 may be the same or different. The positions of the pulsed illumination and the continuous CW illumination in the plasma region 708 may overlap or differ. In some embodiments, the bulb 702 is filled with xenon gas. In some embodiments, the bulb 702 is formed in a spherical shape. Furthermore, in some embodiments, the pressure in the gas-filled bulb 702 ranges from 20 atm to 50 atm.

[0053] Equivalent form

[0054] While the applicant's teachings have been described in conjunction with various embodiments, the applicant's teachings are not limited to such embodiments. Rather, as those skilled in the art will understand, the applicant's teachings encompass various alternatives, modifications, and equivalents that may be made herein without departing from the spirit and scope of the teachings.

Claims

1. An electrodeless laser driving light source, comprising: a) A laser source that generates continuous wave (CW) light at its output end; b) A pump laser that generates pump light at its output; c) A Q-switched laser crystal, the Q-switched laser crystal being positioned to receive the pump light generated at the output of the pump laser, the Q-switched laser crystal generating pulsed laser at the output in response to the generated pump light; d) A first optical element positioned in the path of the pulsed laser, the first optical element projecting the pulsed laser along a first axis onto a breakdown region in a gas-filled bulb that includes ionized gas; e) A second optical element positioned on the path of the CW continuous light, the second optical element projecting the CW continuous light along a second axis onto a CW plasma region in the gas-filled bulb that includes the ionized gas; f) A detector that detects plasma light generated by CW plasma located at least partially in the CW plasma region and generates a detection signal at its output. as well as g) A controller, the input of which is electrically connected to the output of the detector and the output of which is electrically connected to the control input of the pump laser, the controller generating a control signal to control the pump light to the Q-switched laser crystal so as to extinguish the pulsed laser within a delay time after the detection signal exceeds a threshold level.

2. The electrodeless laser-driven light source according to claim 1, wherein the detection signal represents the power of the plasma light, and the threshold level is the desired ratio of the power of the plasma light to the operating power.

3. The electrodeless laser drive source according to claim 2, wherein the desired ratio is nominally 50%.

4. The electrodeless laser drive source according to claim 2, wherein the desired ratio is nominally 90%.

5. The electrodeless laser-driven light source according to claim 2, wherein the desired ratio is in the range of 30% to 95%.

6. The electrodeless laser driving light source according to claim 1, wherein the period between pulses in the pulsed laser is greater than the delay time.

7. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal is configured such that the period between pulses in the pulsed laser is greater than the delay time.

8. The electrodeless laser driving source according to claim 1, wherein the controller is configured such that the delay time is less than one pulse period of the pulsed laser.

9. The electrodeless laser-driven light source of claim 1 further comprises a third optical element positioned in the path of the generated pump light and the path of the generated pulsed laser, wherein the third optical element is configured to separate the generated pump light from the generated pulsed laser.

10. The electrodeless laser driving light source according to claim 9, wherein the third optical element comprises a dichroic element.

11. The electrodeless laser drive source of claim 9, wherein the third optical element is configured to reflect the generated pump light.

12. The electrodeless laser driving light source according to claim 9, wherein the third optical element is configured to transmit the generated pulsed laser.

13. The electrodeless laser driving light source according to claim 1, wherein the first axis and the second axis are coaxial.

14. The electrodeless laser driving light source according to claim 1, wherein the first axis and the second axis are different axes.

15. The electrodeless laser driving light source according to claim 1, wherein the first axis and the second axis are collinear.

16. The electrodeless laser drive source of claim 1, wherein the Q-switched laser crystal is configured such that the pulse repetition rate of the pulsed laser is in the range of 1 kHz to 20 kHz.

17. The electrodeless laser drive source of claim 1, wherein the Q-switched laser crystal is configured such that the pulse repetition rate of the pulsed laser is less than or equal to 1 kHz.

18. The electrodeless laser drive source of claim 1, wherein the Q-switched laser crystal is configured such that the pulse energy of the pulsed laser is in the range of 50 microjoules to 500 microjoules.

19. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal is configured such that the pulse energy of the pulsed laser is in the range of 500 microjoules to 5 microjoules.

20. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal is configured such that the pulse duration of the pulsed laser is in the range of 0.1 ns to 10 ns.

21. The electrodeless laser-driven light source according to claim 1, wherein the laser source is configured such that the power of the CW continuous light is in the range of 5W to 50W.

22. The electrodeless laser-driven light source of claim 1, wherein the laser source is configured such that the power of the CW continuous light is in the range of 5W to 1500W.

23. The electrodeless laser driving light source according to claim 1, wherein the first optical element includes a focusing lens.

24. The electrodeless laser driving light source according to claim 1, wherein the second optical element includes a focusing lens.

25. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal comprises a gain portion and a saturable absorber portion.

26. The electrodeless laser driving light source according to claim 1, wherein the Q-switched laser crystal comprises at least one of a glass body, a yttrium aluminum garnet body, or a spinel body.

27. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal comprises at least one of chromium dopant, cobalt dopant, or vanadium dopant.

28. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal includes a narrowband filter.

29. The electrodeless laser drive source of claim 28, wherein the narrowband filter reflects at least a portion of the plasma light.

30. The electrodeless laser drive source according to claim 28, wherein the narrowband filter blocks wavelengths in the xenon spectrum.

31. The electrodeless laser driving source according to claim 1, wherein the Q-switched laser crystal comprises a coating on one surface.

32. The electrodeless laser drive light source according to claim 1, wherein the gas bulb comprises xenon gas.

33. The electrodeless laser-driven light source according to claim 1, wherein the gas bulb is formed into a spherical shape.

34. The electrodeless laser-driven light source according to claim 1, wherein the pressure in the gas-filled bulb is in the range of 20 atm to 50 atm.

35. A method for illuminating an electrodeless high-brightness plasma light source, the method comprising: a) Using continuous wave (CW) light to provide electromagnetic energy to the gas inside the gas-filled bulb; b) Provide laser pump radiation to the Q-switched laser crystal to generate laser pulses; c) The laser pulse generated by the Q-switched laser crystal is supplied to the gas inside the gas-filled bulb, thereby forming a pulsed plasma in the breakdown region; d) In response to the formation of the pulsed plasma in the breakdown region, the electromagnetic energy supplied to the gas is used to generate CW plasma in the CW plasma region, such that plasma light is emitted from the gas-filled bulb; e) Detect a portion of the emitted plasma light and generate a detection signal; as well as f) After the detected signal exceeds the threshold level, the laser pump radiation to the Q-switched laser crystal is extinguished within a delay time, thereby extinguishing the laser pulse.

36. The method for illuminating an electrodeless high-brightness plasma light source according to claim 35, wherein the generated detection signal represents the power of the plasma light, and the threshold level is a desired ratio of the power of the plasma light to the operating power.

37. The method for lighting an electrodeless high-brightness plasma light source according to claim 35, wherein the breakdown region and the CW plasma region overlap spatially.

38. The method for lighting an electrodeless high-brightness plasma light source according to claim 35, wherein the breakdown region and the CW plasma region are physically separated.

39. The method for lighting an electrodeless high-brightness plasma light source according to claim 35, wherein the delay time is less than one pulse period of the laser pulse.

40. The method for illuminating an electrodeless high-brightness plasma light source according to claim 35, wherein the pulse energy of the laser pulse is in the range of 50 microjoules to 500 microjoules.

41. The method for illuminating an electrodeless high-brightness plasma light source according to claim 35, wherein the pulse energy of the laser pulse is in the range of 500 microjoules to 5 millijoules.

42. The method for illuminating an electrodeless high-brightness plasma light source according to claim 35, wherein the pulse duration of the laser pulse is in the range of 0.1 ns to 10 ns.

43. The method for lighting an electrodeless high-brightness plasma light source according to claim 35, wherein the power of the CW continuous light is in the range of 5W to 1500W.

Images