Superluminal Ring Laser for Hypersensitive Sensing References and Links

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. Ultrahigh enhancement in absolute and relative rotation sensing using fast and slow light, " Phys. Rev. Demonstration of displacement-measurement-sensitivity proportional to inverse group index of intra-cavity medium in a ring resonator, " Opt. Demonstration of a tunable-bandwidth white-light interferometer using anomalous dispersion in atomic vapor, " Phys. Application of fast-light in gravitational wave detection with interferometers and resonators, " J. White-light cavities, atomic phase coherence, and gravitational wave detectors, " Opt. The concept of white light cavities using atomic phase coherence, " Phys. White-light cavity with competing linear and nonlinear dispersions, " Phys. Rev. Anomalous dispersion of transparent atomic two-and three-level ensembles, " Phys. Rev. The modification of the Kronig-Kramers relations under saturation conditions, " Philos. Pump-probe model for the Kramers-Kronig relations in a laser, " to appear in J. Resonance transition 795-nm rubidium laser, " Opt. Simultaneous slow and fast light effects using probe gain and pump depletion via Raman gain in atomic vapor, " Opt. Recently, we have shown theoretically and experimentally that anomalous dispersion can be employed to enhance the mirror displacement sensitivity of a ring resonator, for applications

such as vibrometry, gravitational wave detection and rotation sensing [1][2][3][4].This is accomplished by realizing the so-called White Light Cavity (WLC) [5][6][7][8] which contains a medium with anomalous dispersion [9,10] tuned so that the wavelength becomes independent of frequency.The sensitivity can be defined broadly as S f L ≡ ∂ ∂ , where f is the resonance frequency and L is the length of the cavity.In general, WLC EC S S ξ ≡ where EC S is the sensitivity of the empty cavity, and S WLC is that of the WLC.Under ideal conditions, ξ = 1/n g , where n g is the group index.For vanishing n g , the value of ξ diverges.However, higher order non-linearities within the anomalous dispersion profile prevents the divergence, limiting ξ to a finite value that can be as large as 10 7 for realistic condition.This process is called superluminal enhancement since the group velocity of light far exceeds the free space velocity of light.
However, as discussed previously in Refs 1&3, this enhanced sensitivity does not lead to a corresponding improvement in the minimum measurable mirror displacement or rate of rotation, which is proportional to the linewidth of the cavity.This is because the WLC is broadened by a factor that essentially matches the enhancement in sensitivity.In order to circumvent this constraint, it is necessary to make use of an active cavity, as discussed also in Refs 1&3.To see why, consider, for example, a ring laser gyroscope (RLG).In an RLG, the rate of rotation is proportional to the beat frequency between the two counter-propagating modes.If the RLG is now operated in the WLC mode, the rotation sensitivity will scale by ξ, just as described above.However, the quantum-limited frequency noise, which is proportional to the lifetime of a photon in the cavity, would remain unchanged [1,3].Thus, the minimum measurable rotation rate will improve by a factor of ξ.Variation of this model can be used to measure vibrations, gravitational wave as well as other effects with a similarly enhanced sensitivity, by employing a ring laser in a suitable configuration.
Thus, in order to make use of the anomalous dispersion to enhance the sensitivity of devices such as a gyroscope, a vibrometer or a gravitational wave detector, it is necessary to develop a ring laser that behaves like a WLC: a superluminal laser.At a first glance, it is far from obvious as to how one might realize such a laser.A simple approach might make use of a gain profile with two adjacent peaks as used for realizing a WLC.The dispersion in the zone between the peaks would be negative, and could be tailored to produce the vanishing value of the group index necessary for superluminal propagation.However, there is an apparent problem with this approach.Under the steady state operating condition, the laser gain profile as a function of frequency is flat, with a constant value matching the round trip loss.For frequencies where the gain is less than loss, the gain profile matches the unsaturated one.The overall profile is therefore non-analytic at the boundaries of these regions.Under such a condition, the Karmers-Kronig (KK) relations [11][12][13] cannot be used, and it is not obvious what the dispersion profile would be [14].In this paper, we show how a careful analysis of the behavior of the laser reveals an effective negative dispersion.Specifically, we show that if the gain profile has a dip in the middle, the lasing mode centered at the dip behaves like a WLC, with its frequency becoming highly sensitive to mirror displacement.The enhancement factor can be as high as 1.8 × 10 5 , comparable to what is achievable in a passive cavity.
In what follows, we make use of the semi-classical equation of motion for an effectively single mode ring laser [15].First, we consider light in a ring laser cavity which contains a dispersive medium.The phase and amplitude of the field are described by a set of selfconsistency equations: 2 where φ is the round-trip phase shift, ν is the lasing frequency, and Ω is the resonant frequency of the cavity in the absence of the medium.Ω is given by 2πmc/L where L is the length of the cavity, c is the vacuum speed of light, and m is the mode number.χ′ and χ″ are the real and imaginary part of the susceptibility of the medium, respectively.E is the field amplitude, and Q is the cavity quality factor.ν 0 is the frequency around which χ″ is symmetric.Ω = ν 0 for a particular length of the cavity: L = L 0 .In our model, L will be allowed to deviate from L 0 , thereby making Ω differ from ν 0 .For simplicity, we assume a situation where lasing is unidirectional, made possible by the presence of an optical diode inside the cavity.Any loss induced by the diode is included in Q.
For convenience, we define the parameters, ) subtracting ν 0 from both sides, differentiating with respect to L, and applying d d . By substituting ( )( ) , we get: In order to determine the value of R, it is necessary to know how χ′ depends on ν.In case of a passive cavity, this would be determined solely by the response of the medium to a weak probe, and would be related to χ″ by the KK relations.In the case of an active cavity, however, this is no longer true, since χ′ depends indirectly on E, which varies as a function of ν, as determined by a self-consistent solution of Eq. ( 1).In the later parts of the paper, we explore this interdependence of χ′ and E in detail in order to establish the behavior of the superluminal laser.
We can at this point develop additional insight into the behavior of R by simply assuming that χ′ is antisymmetric around ν = ν 0 (to be validated later).We can then expand χ′ around ν = ν 0 , keeping terms up to (∆ν) 3  where ∆ν = ν−ν 0 .We get (n = 1 or 3).Since χ′ is assumed to be antisymmetric around ν 0 , we have ( ) Differentiating χ′ with respect to ν, and inserting χ′ and the derivative of χ′ in Eq. ( 2), we get: . Here, we have dropped terms that are small due to the fact that 0 ν ν ∆ << .Note that if χ′ is assumed to be linear, corresponding to keeping only the first two terms in the expansion of χ′, the enhancement factor simplifies to: ( )  .To illustrate the physical meaning of this result, we note first that the index can be expressed as ( ) [ ] ( ) . The group index can then be written as ( ) . Thus, the enhancement factor in this linear limit is given simply as R≈1/n g .For normal dispersion (n g >1), R becomes less than one.Hence, the resonant frequency shift with respect to the length variation decreases compared to the shift in an empty cavity.For anomalous dispersion (n g <1), the frequency shift is amplified to be equal to 1/n g times the amount of the shift in the empty cavity.
In order to determine the actual value of R for an active cavity, we need first to establish the explicit dependence of χ′ on the lasing frequency, ν.To this end, we first solve Eq. (1b) in steady state ( 0 Since χ″ is a function of E and ν, the solution to the equation yields the saturated electric field E in steady state inside the laser cavity as a function of the lasing frequency ν.
Let us consider the case in which the cavity contains a medium with a narrow absorption as well as a medium with a broad gain.This configuration creates a gain profile with a dip in the center.The imaginary part of the susceptibility χ″ can then be written as: where = Ω + Γ + − , (k = e or i).We use the subscript "e" for the "envelope" gain profile and "i" for the narrower absorption profile.
Using the modified Kramers-Kronig (MKK) relation [12,13] for the saturated susceptibility, we can then express the real part of the susceptibility χ′ as: ) Here Ω i (Ω e ) is the Rabi frequency equal to is the dipole moment associated with the absorbing (amplifying) medium.The gain and the absorption linewidths are denoted by Γ e and Γ i , respectively.Using the Wigner-Weisskopf model [16] for spontaneous emission, we can define two parameters: ( ) ( ) . In terms of these parameters, the Rabi frequencies can then be expressed as , where ε 0 is the permittivity of free space, and N e and N i represent the density of quantum systems for the absorptive and the amplifying media, respectively.
It is instructive to consider first the case of a conventional gain medium, by setting G i = 0. From Eq. (3), χ′ and χ″ are then simply related to each other as ( ) , it then follows that ( ) ( ) The expression for χ′ is linear in ν, and antisymmetric around ν 0 .Note that according to Eq. (1b), , where Γ c is the empty cavity linewidth.Therefore, we find from R in the linear limit that R 1 g n = , where 1 g c e n = + Γ Γ .Since n g >1, this implies a reduction in sensitivity when compared to an empty cavity.In a typical laser, c e Γ Γ is very small so that this reduction is rather insignificant.The behavior of such a system in the context of the KK relations is discussed in detail in Ref.14, which also addresses the inadequacies of previous studies.
For the case of a conventional laser medium discussed above, it was easy to determine the value of χ′ because of the simple ratio between χ′ and χ″.In particular, this ratio does not depend on the laser intensity.However, for the general case (i.e.G i ≠0), the two terms in χ″ are highly dissimilar.As such, it is no longer possible to find a ratio between χ′ and χ″ that is independent of the laser intensity.Thus, in this case, we need to determine first the manner in which the laser intensity depends on all the parameters, including ν.We define I≡|E| 2 so that G G − > at ν = ν 0 where ε is a small fraction.For a given value of G e , the choice of ε is critical in determining the optimal behavior of χ′.The particular choice of ε = 0.11591 was arrived at via a simple numerical search through the parameter space.Figure 1 shows χ′ as a function of ν.Note first that far away from ν = ν 0 , it agrees asymptotically with the linear value of χ′ for the case of G i = 0, indicated by the dotted line.The inset figure shows an expanded view of χ′.The steep negative slope of χ′ around around ν = ν 0 is the feature necessary to produce the fast light effect (n g ≈0).The black lines shown in Fig. 2 correspond to the exact value of R given by Eq. ( 2).It is also instructive to consider the approximate values of R, where we assume χ′ to be linear.This is shown by the dotted line in inset (a).Of course, this linear approximation is valid only over a very small range around ν = ν 0 .However, it does show clearly that the peak value of R can be understood simply to be due to the linear, negative dispersion at ν = ν 0 .Fig. 2. Sensitivity enhancement associated with Eq. ( 2).The inset (a) shows R in Eq. ( 2) in an expanded view (solid line), and its value in the linear limit (dotted line).The inset (b) is an expanded view of R with a linear vertical scale.
The enhancement factor R indicates how the lasing frequency ν changes when the length of the cavity, L, is changed.It is also instructive to view graphically the dependence of ν on L explicitly.As discussed earlier, in the empty cavity, we have ν 0 = 2πmc/L 0 , where ν 0 is the resonance frequency (chosen to coincide with the center of the dispersion profile).For concreteness, we have used ν 0 = 2π × 3.8 × 10 14 , corresponding to the D 2 transition in Rubidium atoms.We now choose a particular value of the mode number m, so that L is close to one meter.Specifically m = 1282051 yields L 0 = 0.99999978 meter.
When the dispersive gain medium is present, the lasing frequency will be ν = ν 0 if L is kept at L 0 .If L deviates from L 0 , then ν changes to a different value.Specifically, L 2 mc = Ω + from Eq. (1a) in steady state, so that ( ) ( ) Using the dependence of χ′ on ν as shown in Fig. 1, we can thus plot L as a function of ν, as shown in Fig. 3.Even though L is plotted on the vertical axis, this plot should be interpreted as showing how ν changes as L is varied.For ν far away from ν 0 , the variation of ν as a function of L is essentially similar to that of an empty cavity, indicated by the dotted line.Around ν = ν 0 , a small change in L corresponds to a very big change in ν as displayed in the inset figure.Specifically, we see that ∆L≈10 −13 meter produces f≡∆ν/(2π)≈10 5 Hz, corresponding to ∆f/∆L~10 18 .In contrast, for a bare cavity, ∆L≈2 × 10 −7 produces ∆ν/(2π)≈8 × 10 7 Hz, corresponding to ∆f/∆L~4 × 10 14 .The value of ∆f/∆L for a conventional laser is also very close to this value, as indicated by the convergence of the dotted and solid lines for ν far away from the superluminal region.Thus the enhancement factor, R, is ~2.5 × 10 3 .If we zoom in even more, we will eventually see that as ∆L→0, we have R~1.8 × 10 5 , as shown in Fig. 2.  The scheme proposed here for a superluminal laser may be realizable in many different ways.For example, the gain profile can be produced by a diode pumped alkali laser [17].The dip in the gain profile can be produced by inserting a Raman probe, following the technique shown in Ref. 18. Figure 4 displays the corresponding energy levels, and the experimental configuration to realize a superluminal laser.presence of a buffer gas of 4 He [17].Figure 4(b) shows the enegy levels within the D1 transition used for producing the Raman depletion.The lasing beam acts as the Raman pump.The Raman probe is produced by sampling a part of the laser output, and shifting it in frequency with an acousto-optic modulator, operating at the frequency that matches the hyperfine splitting in the ground state [18].An additional optical pumping beam is applied to produce the population inversion among the metastable hyperfine states necessary for the Raman gain and depletion.Thus, the laser light experiences the Raman dip in the cell B, as illustarted in fig.(c).Efforts are underway in our laboratory to realize such a scheme.
To summarize, we have shown that the effective dispersion experienced by a laser frequency is indirectly sensitive to the spectral profile of the unsaturated gain, even though the saturated gain becomes flat, equaling the cavity loss.Specifically, we have shown that a dip in the gain profile when properly tuned, 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 with a wide range of sensing application, including gyroscopy, vibrometry and gravitational wave detection.We also outline an experimental scheme for realizing such a superluminal laser.
frequency shifts under a perturbation of L in the empty and filled cavity, respectively.We consider the ratio, amount of the frequency shift is enhanced (R>1) or diminished (R<1) by the intracavity medium.To derive R, we begin with Eq. (1a).In steady state ( 0 ϕ = ɺ Eq. (3a) equal to −1/Q, we get aI 2 + bI + c≡0, where a, b, and c are function of various parameters.We keep the solution that is positive over the #129230 -$15.00USD Received 28 May 2010; revised 23 Jul 2010; accepted 25 Jul 2010; published 2 Aug 2010 (C) 2010 OSA 16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17661 solution for I to evaluate Ω i 2and Ω e 2 , in Eq. (3b) we get an analytic expression for χ′.The behavior of χ′ can be understood by plotting it as a function of ν.For illustration, we consider Γ e = 2π × 10 9 s −1 , Γ i = 2π × 10 7 s −1 , ν 0 = 2π × 3.8 × 10 14 s −1 , N e = 9 × 10 6 , and N i = 1.2645 × 10 11 .To fulfill the lasing condition in the spectral range of the absorption dip, we consider the gain peak 12 Q

Fig. 1 .
Fig. 1.Real part of the steady-state susceptibility for the combined absorbing and amplifying media (solid line), and the conventional gain medium (dashed line).The inset shows an expanded view of the solid line in the main figure We can now evaluate the enhancement factor, R, as expressed in Eq. (2).first that ( )( ) I I d d χ ν χ ν χ ν ′ ′ ′ = ∂ ∂ + ∂ ∂ ∂ ∂ , where I ν ∂ ∂ is evaluated from the solution of the

Fig. 4 .
Fig. 4. Energy levels for (a) 795-nm Rb laser to produce broadband gain, (b) Raman depletion to induce narrowband absorption dip.(c) Schematics of the experimental set-up to realize a superluminal laser: PBS, polarizing beam splitter; BS, beam splitter; AOM, acousto-optic modulator.Note that the superluminal laser is the same as the Raman pump.The scheme shown is for 85 Rb atoms.The broadband gain is produced by side-pumping with a diode laser array.

Figure 4 (
a) illustrates the atomic transitions associated with the process for producing a broadband gain profile.The diode pump, applied from the side along the D2 transition, induces population inversion for the D1 transition in the #129230 -$15.00USD Received 28 May 2010; revised 23 Jul 2010; accepted 25 Jul 2010; published 2 Aug 2010 (C) 2010 OSA 16 August 2010 / Vol. 18, No. 17 / OPTICS EXPRESS 17664