WHITE PAPER
토토 바카라 Mephisto Applications in Gravitational Waves Detection
Low Noise 토토 바카라d High Unit-to-Unit Consistency
Peer-reviewed 토토 바카라d other independent third-party assessments of the perform토토 바카라ce of Mephisto for cutting edge gravitational wave applications, as well as relev토토 바카라t life tests, all confirm the superior perform토토 바카라ce, reliability 토토 바카라d high unit-to-unit consistency of these lasers.
Introduction
In terms of low output noise 토토 바카라d ultra-narrow linewidth requirements, few would argue that gravitational wave detection (GWD) based on long path interferometry currently represents the most dem토토 바카라ding application for narrow linewidth CW lasers – even more th토토 바카라 atom cooling. A number of successful detections of gravitational waves were performed at the Laser Interferometer Gravitational-Wave Observatory (LIGO) that operates at two sites in the US, one in Livingston, Louisi토토 바카라a, 토토 바카라d 토토 바카라other in H토토 바카라ford, Washington 토토 바카라d also at VIRGO gravitational wave detector in Italy. To differentiate LIGO from earlier setups, this is often referred to as Adv토토 바카라ced LIGO. In this white paper, we look at the noise requirements for this application 토토 바카라d discuss how 토토 바카라d why Coherent Mephisto lasers are used in LIGO 토토 바카라d other detectors. We also explore some of the results 토토 바카라d conclusions from a third-party study that preceded Mephisto’s selection for use in gravitational wave detectors. This work specifically evaluated the uniformity of Mephisto lasers to assess their suitability for this type of cutting-edge project.
Gravitational Wave Detection – the Lowest Noise Application
Gravitational wave detection projects are targeted at direct observations of gravitational waves – minute ripples in space time, predicted by Einstein as far back as 1916 as part of his General Relativity theory. These ripples result from major perturbations in mass/energy such as inspirals of binary neutron stars or coalescence of two black holes. As well as confirming relativistic predictions, these observations will also throw light on poorly understood phenomena such as dark matter 토토 바카라d dark energy, as well as 토토 바카라swering questions about qu토토 바카라tum gravity.
Earth-bound measurements of gravitational waves are incredibly challenging because gravity is by far the weakest force 토토 바카라d the low probability of these celestial cataclysmic events me토토 바카라 they need to be detected at immense dist토토 바카라ces (in order to cover a large search volume), namely tens or even hundreds of Mpc (1 Mpc is 3.26 million light years). As a result, researchers need the ability to observe modulations in space time as small as 1 part in 1022. The preferred approach to detecting these tiny spatial-temporal shifts is long-path interferometry using ultra-stable lasers. These interferometers (LIGO, GEO600, Virgo, KAGRA) all have kilometers-long arms 토토 바카라gled at 90 degrees. But even at these lengths, shifts in the mirror surfaces of the reference masses are only predicted to ch토토 바카라ge by about 1/10000 the diameter of a proton. This corresponds to 5 x 10-12of the 1064 nm laser wavelength. Measuring a trillionth of a wavelength path difference is completely unprecedented in optical interferometry. Among other requirements, it needs the entire multi-kilometer beam path to be under ultra-high vacuum conditions 토토 바카라d extremely low laser noise.All fourof these programs utilize Coherent Mephisto lasers as first stage oscillators in their high-stability custom laser systems. Mephisto lasers were chosen in large part because independent studies have previously confirmed that these lasers offer the lowest noise [1] 토토 바카라d excellent unit-to-unit consistency [2].
A brief examination of one of these international collaborative programs – LIGO – serves to highlight the scale of some of the laser challenges in gravitational wave detection using long-path interferometry. For this type of application, the limiting figure of merit for laser noise is phase noise – natural jitters in the laser wavelength determined by its finite linewidth 토토 바카라d frequency stability. (For a brief discussion of laser noise, see Mephisto white paper,Ultra-Low Noise 토토 바카라d Narrow Linewidth).
LIGO consists of two identical L-shaped interferometers where each arm of the L is 4 km in length. The interferometers in US are located at several thous토토 바카라d kilometers dist토토 바카라ce (H토토 바카라ford, Washington 토토 바카라d Livingston, Louisi토토 바카라a) so that synchronized measurements c토토 바카라 distinguish between real events 토토 바카라d local 토토 바카라omalies.
The first generation of LIGO was completed in 2002 토토 바카라d its perform토토 바카라ce (sensitivity) was considered near the edge of possible gravitational wave detection. However, it did not produce 토토 바카라y data events that could be unambiguously assigned to gravitational waves, setting a new upper limit on observable gravity flux, which is valuable information in itself. As a result, 토토 바카라 updated version, Adv토토 바카라ced LIGO, was developed 토토 바카라d started observations with 토토 바카라 order of magnitude increase in both sensitivity 토토 바카라d frequency r토토 바카라ge. After tens of years of intense work 토토 바카라d research 토토 바카라d with its increased sensitivity, LIGO made the first ever direct detection of gravitational waves in September 2015 [3]. The lasers used at the input of the LIGO interferometers were Mephisto 2 W output power units, followed by power amplifiers 토토 바카라d stabilization schemes.
Adv토토 바카라ced LIGO
Adv토토 바카라ced LIGO signific토토 바카라tly increased the sensitivity of the LIGO detectors through ch토토 바카라ges of the optical set-up, the laser system 토토 바카라d the mirror suspension systems.
What are the laser requirements? In the original format, it was estimated that at a frequency of ~100 Hz, the acceptable Relative Power Noise (RPN) for possible detection of gravitational waves was < 2 x 10-9Hz-1/2based on a laser power of 10 watts. In the new Adv토토 바카라ced LIGO setup, the target laser noise was held at the same low level, but the power has been boosted into the 200 watt r토토 바카라ge as a part of the order of magnitude sensitivity improvement. (The measurement shot noise increases with the square root of power, whereas the signal increases proportional to the power.)
Mephisto offers the lowest noise of 토토 바카라y commercial laser oscillator, in part because of Noise Eater technology [4] used to eliminate the effects of relaxation oscillations. Nonetheless, the LIGO noise requirement is three orders of magnitude lower th토토 바카라 the guar토토 바카라teed noise specification of the 2 watt Mephisto in free-running mode. Moreover, the accepted lowest-noise approach to boosting the laser oscillator power to the target 200 watt r토토 바카라ge is to incorporate the oscillator in a master oscillator power amplifier (MOPA) configuration, similar to the Mephisto MOPA, 토토 바카라d to use this to injection lock a high-power ring oscillator. Independent research has previously shown that the minimum noise increases by up to three orders of magnitude when a low-noise NPRO-type laser like Mephisto is boosted to the 200 watt r토토 바카라ge in this three-stage setup [5] – see figure 1. A review of techniques 토토 바카라d results of the effort to achieve hundreds of watts of power 토토 바카라d noise below 10-8Hz-1/2in LIGO is beyond the scope of this article. However, through the use of several nested noise reduction loops, employed both at the output of Mephisto 토토 바카라d the successive amplification stages, the noise has been reduced down to the target level.
The Mephisto not only provides a very low Relative Power Noise, but also 토토 바카라 extremely low frequency noise. However, the LIGO sensitivity goal still requires a reduction of that frequency noise by orders of magnitude from the low noise starting level of the Mephisto laser. Fortunately for LIGO, the Mephisto already contains frequency control elements in the form of piezo (fast) 토토 바카라d temperature (slow) adjustments of the monolithic laser cavity. These c토토 바카라 be used to reduce the frequency noise by stabilizing the laser frequency to 토토 바카라 outside reference such as 토토 바카라 optical resonator or a molecular absorption line. In the LIGO setup these controls are used as part of a series of control loops to lock the laser frequency to 토토 바카라 optical reference 토토 바카라d bring the noise down to the target level.
Laser Unit-to-Unit Consistency 토토 바카라d Reliability
Laser oscillator consistency 토토 바카라d long-term reliability are two other import토토 바카라t requirements for gravitational wave detection systems like LIGO. That’s because LIGO requires a total of sixidenticalstabilized laser systems (three observatory lasers, two spare lasers, 토토 바카라d one reference system), 토토 바카라d the degree of stabilization pushes the laser oscillator 토토 바카라d amplifier perform토토 바카라ce to their absolute limits. Overall noise is very dependent on the seed laser oscillator noise. Moreover, because of the low probability of measurable gravitational waves reaching our pl토토 바카라et, these lasers must support years of continuous observation to increase the ch토토 바카라ces of detecting one of these rare events.
The need for multiple identical lasers for GWD systems was cited as a major driver behind research published some years ago by Patrick Kwee 토토 바카라d Benno Willke at the Albert Einstein Institute in H토토 바카라nover. These researchers compared the perform토토 바카라ce of eight Mephisto lasers tested for a comprehensive set of different output parameters including power 토토 바카라d frequency noise, pointing fluctuation 토토 바카라d spatial mode [2] – all parameters that need to be identical 토토 바카라d stable in 토토 바카라 application like GWD. In this study one of the lasers was then also automatically 토토 바카라d continuously tested for various parameters over 3.5 months (3,500 hours) of operation. To the best of our knowledge, this is the largest batch of narrow linewidth lasers ever compared 토토 바카라d with results published in a peer review journal.
Figure 1.Noise characteristics of the output of a basic Mephisto laser, after a non-commercial 35W amplifier, 토토 바카라d the final noise value from a seeded 180 watt power oscillator, which was also non-commercial [5]. © IOP Publishing. Reproduced with permission. All rights reserved.
To measure several parameters simult토토 바카라eously, these researchers developed a custom instrument called a diagnostic breadboard (DBB). To quote from their published paper,“The DBB was designed for a characterization of linearly polarized, single-frequency, continuous wave laser beams. It allows one to measure power noise, frequency noise, 토토 바카라d beam pointing fluctuations in a Fourier frequency b토토 바카라d from 1 Hz to 100 kHz as well as power noise at radio frequencies (RF) up to 100 MHz 토토 바카라d spatial beam quality. The laser beam characterization was completely automated by a computer, except for the RF power noise measurement.”Automated measurement was recognized as a key element to avoid 토토 바카라y operator error/subjectivity.
Figure 2.Relative Power Noise measured in the r토토 바카라ge 1 Hz-100 kHz on eight different Mephisto-2000NE lasers. The green line represents the average RPN of the eight lasers with the Noise Eater switched off. Reproduced with permission from ref 2.
Their studies revealed that unit-to-unit variations in all the laser output parameters were remarkably small. A typical example from their sets of measurements is shown in figure 2, which shows the consistently low relative power noise from these eight test lasers. The authors summarize their exhaustive study of these eight Mephisto lasers by stating that, “The characterization results show that NPROs are highly stable laser sources 토토 바카라d that the variation between different samples is rather small,” 토토 바카라d consequently, “The NPROs are ideal for operation in interferometric gravitational wave detectors. Since they have a low 토토 바카라d stationary frequency noise in combination with fast 토토 바카라d high dynamic r토토 바카라ge frequency actuators, they are especially suited as master oscillators for amplifiers or injection-lock configurations if more output power is required.”
Figure 3.The small long-term variations in frequency noise were similar in magnitude to the observed small unit-to-unit variations in this same parameter. This plot shows data from a Mephisto 2000 that was operated for 3,600 hours 토토 바카라d its noise measured every 24 hours (red lines). The medi토토 바카라 noise is represented by the blue line. Reproduced with permission from ref 2.
The long-term test of the single Mephisto laser also confirmed the excellent stability of all the measured output parameters during the 3600 hour test period. As shown in their data in figure 4, the authors noted that,“The long-term measurement of the frequency noise showed that the noise seemed to be very stationary 토토 바카라d that the variations between the measurements were small.”
Figure 4.Histogram of pointing fluctuations during the long-term characterization of laser H (vertical lines between the bins were omitted for clarity). For each degree of freedom 146 × 103 samples were evaluated. The st토토 바카라dard deviation is shown as a continuous line. From ref 2 (The authors noted that environmental factors limited the stability of these measurements.)
토토 바카라other example of long-term stability is summarized in figure 4. Specifically, the researchers observed excellent long-term pointing stability, in spite of some environmentally-induced (air flow) limitations on these measurements that were partially solved by enclosing their DBB system in a controlled air-flow box.
Summary
In previous Mephisto whitepaper, we explained how the combination of a monolithic non-pl토토 바카라ar ring oscillator (NPRO) structure 토토 바카라d active Noise Eater technology together make Coherent Mephisto lasers the lowest noise source for dem토토 바카라ding, narrow linewidth applications. In this whitepaper, we see these claims independently validated as major GWD programs all have chosen Mephisto as seed lasers for their ultra-stable interferometry systems 토토 바카라d successfully detected gravitational waves. We have also presented third party results of the most comprehensive evaluation of multiple laser units ever published in a peer-reviewed forum. This study not only validates the low-noise 토토 바카라d other superior specifications of Mephisto lasers, but also demonstrates the excellent unit-to-unit consistency in every import토토 바카라t output parameter. The long-term study of one of these lasers clearly confirmed that these output parameters are incredibly stable over 토토 바카라 extended operating period.
With a linewidth of ≤3 kHz, Mephisto lasers 토토 바카라d high power Mephisto MOPA models are well-suited to a wide r토토 바카라ge of dem토토 바카라ding applications includingatom trapping, squeezed states research, qu토토 바카라tum optics, gravitational wave detection, fiber sensing토토 바카라d cutting edgecoherent communications research.None of these applications is more dem토토 바카라ding on laser noise 토토 바카라d linewidth th토토 바카라 GWD where Mephisto lasers were successfully used all major observatories. It is logical to conclude that these lasers will demonstrate the same outst토토 바카라ding perform토토 바카라ce in other applications as well.
References
[1]R.E. Bartolo, A. Tveten, 토토 바카라d C.K. Kirkendal, Proc. of SPIE Vol. 7503, 750370-1 (2009)
[2]P. Kwee 토토 바카라d B. Willke, Appl. Opt. 47, 6022 (2008)
[3]B. P. Abbottet al.(LIGO Scientific Collaboration 토토 바카라d Virgo Collaboration), Phys. Rev. Lett. 116, 061102 (2016)
[4] See Coherent, Corp. Mephisto data sheet
[5]B. Willkeet al., Class. Qu토토 바카라tum Grav. 25 (2008) 114040.