Lack of Practical Deep UV Lasers has Limited UV Raman Spectroscopy
A major reason that UV Raman spectroscopy has not yet found a major place in the world of analytical instrumentation has been the availability of compact, cost effective deep UV lasers.
Frequency Doubled Ion Lasers
Present deep UV lasers of choice for UV Raman spectroscopy are frequency doubled argon or krypton lasers that provide a wide range of deep UV wavelengths, CW output, and single transverse mode operation. Although these are all very desirable traits for UV Raman spectroscopy, these lasers cost in the order of $100,000, consume over 12,000 W of electric power, require water cooling and are bulky and heavy. Cost is often not the major impediment to the use of these lasers. It is the lack of mobility and cost of installation and operation. Another laser of potential use for
4th Harmonic DPSS
UV Raman is the 266nm, 4th harmonic DPSS laser. This laser, although attractive for some applications, suffers from fluorescence interference in many organic and inorganic materials for Raman bands above about 1200 cm-1. In addition, 266nm is not an ideal wavelength for matching the resonance bands of many materials, and the peak power and low duty cycle of these lasers is problematic for probing many organic molecules without thermal or photochemical damage.
Practical Deep UV: Photon Systems Deep UV Hollow Cathode Lasers
Photon Systems new family of deep UV lasers offer emission wavelengths at 224.3nm, 248.6nm, and other potential wavelengths at 260nm, 270nm and others. These lasers are the size, weight, and power consumption of HeNe lasers. And they have the following attributes:
- > 100 mW, quasi-CW output at several deep UV wavelengths (to over 500mW)
- Square wave laser output pulses with adjustable pulse width: <20 μs to >300 μs
- Pulse repetition rates up to 1kHz or more (limited by long term average power)
- Narrow emission linewidth: <3GHz, (i.e. < 0.1 cm-1 or <0.0005nm)
- Ultra stable frequency: <1 ppm (totally independent of temperature)
- Instantaneous warm-up from any ambient temperature: <20 μs
- No standby, preheating power or time required
- Wide ambient temperature range: +120 C to –150 C
- Compact laser tube: as small as 15cm long by 3.8cm diameter
- Compact power supply: 5cm wide x 15 cm long x 3 cm high
- Low power consumption: can be less than 1 W (can be powered by USB port)
- Low cost: OEM prices less than $4000
- Long lifetime: >2000 hours depending on usage (discussed in more detail below)
- No toxic materials
Although these lasers have many great attributes, they have limitations that include:
- Lifetime (discussed below) is pulse dependent. The lower the pulse repetition rate, the
longer the lifetime.
- The “times diffraction limit”, or M2 value of the laser is about 10, which means the laser
typically cannot be focused to spot sizes less than about 3 μm.
- The lasers are not true CW.
Basic Operation of HeAg and NeCu Lasers
We describe the output of our lasers as “quasi-CW” for the following reason. The laser transitions are CW transitions. This means that as long as pumping is maintained above threshold, there is laser output.
Theoretically the output power can be truly CW with no interruptions for hours or months. The input power to maintain pumping above threshold and provide true CW output varies between 3kW and 10kW, depending on the laser transition. The slope efficiency of these lasers is high such that if pumping above threshold occurs, the laser output power increases rapidly. Typically output power is over 100mW at any of the deep UV wavelengths from a laser tube less than 50cm in length.
In order to keep the lasers small, simple and inexpensive, we have chosen to limit the long-term average input and output power by commutating (chopping) the input power with a duty cycle less than a few percent. Commutated operation is possible with our type of laser because of the fast time constant between application of voltage and laser output associated with our transverse hollow cathode glow discharge design. This cannot be done with positive column laser designs such as argon or krypton ion lasers, helium neon, or helium cadmium lasers. We can adjust the width of the drive power pulse to the laser tube in a range from a few μs to hundreds of μs. And we can adjust the pulse repetition rate from single pulses to over 1000 Hz. The rise and fall time of the output of the laser is typically about 15 μs to 20 μs. Electrical energy input to the laser tube in a 100 μs long pulse is about 0.5J. This is illustrated below. When a laser is operating at 1Hz, the input power is only about 0.5W. The power needed to maintain the laser above threshold during its “on” time, is supplied from a capacitor. Therefore, operation is similar to a flashlamp, except that the pulse width is controllable and more constant. We typically do not operate our lasers above about 3% duty cycle, which corresponds to a pulse repetition rate (PRF) of 300 Hz. Higher duty cycles are possible but only for limited periods of time, determined by thermal overheating of the laser tube or electronic components.
Hollow Cathode Laser Lifetime
In order to have a viable commercial product, it is our belief that the laser lifetime must exceed about one year of useful “field” use before requiring maintenance, repair or replacement. The “field lifetime” of the laser is strongly dependent on the method of use of the laser. This will be discussed below.
In our development of 224nm HeAg and 248nm NeCu lasers we have identified three basic lifetime limitation mechanisms within the laser tubes: bore erosion, buffer gas cleanup and mirror contamination. These lasers employ a hollow cathode glow discharge to form the gain medium for lasing. This technology is similar to hollow cathode lamps wherein a basic lifetime limit is related to sputter erosion of the hollow metal cathode. As metal from the inside diameter of the cathode is sputtered away, several ageing processes occur: physical change in the shape of the hollow cathode, trapping of buffer gas under the sputter deposits, and contamination of laser mirror surfaces. Each of these processes is related to the product of drive (discharge) current and time. Since our lasers are operated in a commutated fashion, the laser is kept “on” for the shortest possible time needed to make a useful measurement. This enables the longest possible operating lifetime of the laser. Operating with 100 μs pulse width at a pulse repetition frequency (PRF) of 100 Hz, the bore erosion lifetime is between about 1500 and 3000 hours. With the same pulse width but operating at a PRF of 1 Hz, the bore erosion lifetime is expected to be over 100,000 hours or essentially infinite. This corresponds to over 500 million, 100 μs wide, pulses. If the pulse width is reduced, the number of pulses is increased.
The buffer gas lifetime depends of the method of replacement of buffer gas trapped under sputter deposits. Photon Systems has two types of methods: passive and active buffer gas pressure control. In the passive method the laser tube and/or an added passive gas ballast provides the gas needed for the required lifetime of the laser. For applications where the PRF is low and data are accumulated on a single pulse basis, this passive regulation method is desirable because it is the simplest and least expensive. When higher average power is needed or more total amount of pulses are needed or required of the laser, an active buffer gas pressure regulation system is employed. In this system we sense the gas pressure and using a double solenoid valve, “burping”, system we maintain the buffer gas pressure constant. The active pressure regulation system can eliminate the buffer gas lifetime issue. The passive ballast lifetime depends on the usage environment of the laser and the amount of passive gas ballast.
Mirror contamination lifetime is the primary lifetime problem with our present lasers. We have demonstrated lifetimes over 1000 hours operating at 1 Hz with a 25% degradation in output. At 50% degradation in output the lifetime is several thousand hours. Put in pulse terms, the 25% degradation point is over 3 million pulses and 50% degradation is over 11 million pulses. Output from our lifetests seems to level off after about the 50% degradation point, so it is not presently clear what the lifetime is to 25% of initial output. It may very well be over 30 million pulses. We continue to work on mirror contamination issues and believe this lifetime limitation will continue to improve with time. An example of the lifetime curve for a 248nm laser is shown below.
In most cases a laser tube can be reprocessed at low cost when mirror contamination or buffer gas pressure lifetime limits are reached. This can typically be done more than 10 times before the ultimate bore erosion lifetime limit of a tube is reached. Reprocessing involves putting the laser on a vacuum process station, removing and cleaning laser mirrors, remounting mirrors and performing a vacuum reprocessing of the tube. Reprocessing time is less than one day.
Since these lasers come to full output within about 20 μs of demand for laser output, none of the laser lifetime is wasted on warm-up. Warm-up is a major consumer of lifetime of competitive laser technologies.
Hollow Cathode OEM Laser Cost
The inherent cost of these lasers is low. Basic laser tube prices can be less than $3000 with power supply cost less than $1000 in OEM quantities. We believe the biggest barrier to commercial application of these lasers is not cost or price. It is making the right marriage between this laser source technology and a detection technology that is useful for some analytical instrument application. Ultimately this means using the laser at low PRF or using the laser is short bursts.
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1512 Industrial Park St. Covina, CA 91722-3417