J. Yablon

Superluminal ring laser for hypersensitive sensing

The group velocity of light becomes superluminal in a medium with a tuned negative dispersion, using two gain peaks, for example. Inside a laser, however, the gain is constant, equaling the loss. We show here that the effective dispersion experienced by the lasing frequency is still sensitive to the spectral profile of the unsaturated gain. In particular, a dip in the gain profile leads to a superluminal group velocity for the lasing mode. The displacement sensitivity of the lasing frequency is enhanced by nearly five orders of magnitude, leading to a versatile sensor of hyper sensitivity.

Theoretical modeling and experimental demonstration of Raman probe induced spectral dip for realizing a superluminal laser

We have demonstrated experimentally a Diode-Pumped Alkali Laser (DPAL) with a Raman resonance induced dip in the center of the gain profile, in order to produce an anomalous dispersion, necessary for making the laser superluminal. Numerical calculations match closely with experimental results, and indicate that the laser is operating superluminally, with the group index far below unity (~0.00526) at the center of the dip. The estimated factor of enhancement in the sensitivity to cavity length perturbation is ~190, approximately equaling the inverse of the group index. This enhancement factor can be made much higher via optimal tuning of parameters. Such a laser has the potential to advance significantly the field of high-precision metrology, with applications such as vibrometry, accelerometry, and rotation sensing.

Modeling and analysis of an ultra-stable subluminal laser

Abstract We describe a subluminal laser which is extremely stable against perturbations. It makes use of a composite gain spectrum consisting of a broad background along with a narrow peak. The stability of the laser, defined as the change in frequency as a function of a change in the cavity length, is enhanced by a factor given by the group index, which can be as high as 105 for experimentally realizable parameters. We also show that the fundamental linewidth of such a laser is expected to be smaller by the same factor. We first present an analysis where the gain profile is modeled as a superposition of two Lorentzian functions. We then present a numerical study based on a physical scheme for realizing the composite gain profile. In this scheme, the broad gain is produced by a high pressure buffer-gas loaded cell of rubidium vapor. The narrow gain is produced by using a Raman pump in a second rubidium vapor cell, where optical pumping is used to produce a Raman population inversion. We show close agreement between the idealized model and the explicit model. A subluminal laser of this type may prove to be useful for many applications.

Effect of multiorder harmonics in a double-Raman pumped gain medium for a superluminal laser

Abstract. Use of a double-Raman pump applied to a three-level system is a convenient method for generating negative dispersion. When the gain at the center is high enough, such a system can be used to realize a superluminal laser, which in turn can be used to enhance the sensitivity of rotation sensors. For this condition, it is often necessary to apply strong pumps that are closely spaced in frequency. Accurate modeling of this system thus requires taking into account interference between the two pumps. We present such an analysis where we allow for an arbitrary number of harmonics that result from this interference, and investigate the behavior of the gain profile under a wide range of conditions. We also describe an experimental study of double-Raman gain in a Rb vapor cell, and find close agreement between the experimental result and the theoretical model. The technique reported here can be used in developing a quantitative model of a superluminal laser under wide-ranging conditions.

Demonstration of a highly subluminal laser with suppression of cavity length sensitivity by nearly three orders of magnitude

We have demonstrated a laser in which the frequency shift due to small cavity fluctuations is far less than what would be expected from a conventional laser. The factor of sensitivity suppression is inferred to be equal to the effective group index experienced by the laser, implying that this laser is subluminal. We have observed a suppression factor as high as 663. Such a laser is highly self-stabilized compared to a conventional laser, and is expected to have a far smaller Schawlow-Townes linewidth. As a result, this laser may have potentially significant applications in the fields of high-precision optical metrology and passive frequency stabilization.

High-depth-resolution range imaging with multiple-wavelength superheterodyne interferometry using 1550-nm lasers

Lasers and laser diodes are widely used as illumination sources for optical imaging techniques. Time-of-flight (ToF) cameras with laser diodes and range imaging based on optical interferometry systems using lasers are among these techniques, with various applications in fields such as metrology and machine vision. ToF cameras can have imaging ranges of several meters, but offer only centimeter-level depth resolution. On the other hand, range imaging based on optical interferometry has depth resolution on the micrometer and even nanometer scale, but offers very limited (sub-millimeter) imaging ranges. In this paper, we propose a range imaging system based on multi-wavelength superheterodyne interferometry to simultaneously provide sub-millimeter depth resolution and an imaging range of tens to hundreds of millimeters. The proposed setup uses two tunable III-V semiconductor lasers and offers leverage between imaging range and resolution. The system is composed entirely of fiber connections except the scanning head, which enables it to be made into a portable device. We believe our proposed system has the potential to tremendously benefit many fields, such as metrology and computer vision.

Ultra-precise rotation sensing with a superluminal ring laser

We show that group velocity of light far exceeding the vacuum speed can be realized when a medium that produces a narrow band dip in the gain profile is placed inside a ring laser. The dip leads to an effective negative dispersion, which can be tuned to produce a very small group index. The rotational sensitivity of the ring laser is enhanced by a factor equaling the inverse of the group index. For a realistic system, the enhancement factor can be as high as 1.8*105. In order to realize such a device, the background gain can be produced by using, for example, an optically pumped Ti:Sapphire crystal, a semiconductor optical amplifier, or a diode pumped alkali laser. The narrow dip can be produced, for example, by a Rb cell configured for Raman depletion. Here, we present the theoretical model behind such a superluminal laser, and describe a preliminary experiment for realizing such a device.

A subluminal laser with extreme insensitivity to cavity length change

With a narrow peak on top of a broad-band gain profile in a laser, the group-velocity of light becomes significantly subluminal. The lasing frequency becomes extremely insensitive to cavity length change in such a laser.

Double-Raman gain for realizing a superluminal ring laser

We show how a dual-peak gain produced via optical pumping and non-degenerate Raman pumps in a single vapor cell can be used to realize a superluminal ring laser with enhanced sensitivity for gyroscopy and accelerometry.

Observation of Raman resonance of a probe in a Rb cell added to an Ethane-Rb laser for realizing a superluminal laser

We report sub-natural linewidth Raman resonance for a probe applied to an auxiliary Rb cell added to an Ethane-Rb laser as a key step towards realizing a superluminal ring laser for ultrasensitive gyroscopy and accelerometry.