1-GHz dual-comb spectrometer with high mutual coherence for fast and broadband measurements.

Dual-frequency comb spectroscopy permits broadband precision spectroscopy with high acquisition rate. The combs' repetition rates as well as the mutual coherence between the combs are key to fast and broadband measurements. Here, we demonstrate a 1-GHz high-repetition-rate dual-comb system with high mutual coherence (sub-Hz heterodyne beatnotes) based on mature, digitally controlled, low-noise erbium-doped mode-locked lasers. Two spectroscopy experiments are performed with acquisition parameters not attainable in a 100-MHz system: detection of water vapor absorption around 1375 nm, illustrating the potential for fast and ambiguity-free broadband operation, as well as acquisition of narrow gas absorption features across a spectral span of 0.6 THz (600 comb lines) in only 5 µs.

Broadband near-infrared astronomical spectrometer calibration and on-sky validation with an electro-optic laser frequency comb.

The quest for extrasolar planets and their characterization as well as studies of fundamental physics on cosmological scales rely on capabilities of high-resolution astronomical spectroscopy. A central requirement is a precise wavelength calibration of astronomical spectrographs allowing for extraction of subtle wavelength shifts from the spectra of stars and quasars. Here, we present an all-fiber, 400 nm wide near-infrared frequency comb based on electro-optic modulation with 14.5 GHz comb line spacing. Tests on the high-resolution, near-infrared spectrometer GIANO-B show a photon-noise limited calibration precision of < 10 cms as required for Earth-like planet detection. Moreover, the presented comb provides detailed insight into particularities of the spectrograph such as detector inhomogeneities and differential spectrograph drifts. The system is validated in on-sky observations of a radial velocity standard star (HD221354) and telluric atmospheric absorption features. The advantages of the system include simplicity, robustness and turn-key operation, features that are valuable at the observation sites.

All-optical stabilization of a soliton frequency comb in a crystalline microresonator.

We demonstrate the all-optical stabilization of a low-noise temporal soliton based microresonator based optical frequency comb in a crystalline resonator via a new technique to control the repetition rate. This is accomplished by thermally heating the microresonator with an additional probe laser coupled to an auxiliary optical resonator mode. The carrier-envelope offset frequency is controlled by stabilizing the pump laser frequency to a reference optical frequency comb. We analyze the stabilization by performing an out-of-loop comparison and measure the overlapping Allan deviation. This all-optical stabilization technique can prove useful as an actuator for self-referenced microresonator frequency combs.

Mode spectrum and temporal soliton formation in optical microresonators.

The formation of temporal dissipative solitons in optical microresonators enables compact, high-repetition rate sources of ultrashort pulses as well as low noise, broadband optical frequency combs with smooth spectral envelopes. Here we study the influence of the microresonator mode spectrum on temporal soliton formation in a crystalline MgF2 microresonator. While an overall anomalous group velocity dispersion is required, it is found that higher order dispersion can be tolerated as long as it does not dominate the resonator's mode structure. Avoided mode crossings induced by linear mode coupling in the resonator mode spectrum are found to prevent soliton formation when affecting resonator modes close to the pump laser frequency. The experimental observations are in excellent agreement with numerical simulations based on the nonlinear coupled mode equations. The presented results provide for the first time design criteria for the generation of temporal solitons in optical microresonators.

A Kerr polarization controller.

Kerr-effect-induced changes of the polarization state of light are well known in pulsed laser systems. An example is nonlinear polarization rotation, which is critical to the operation of many types of mode-locked lasers. Here, we demonstrate that the Kerr effect in a high-finesse Fabry-Pérot resonator can be utilized to control the polarization of a continuous wave laser. It is shown that a linearly-polarized input field is converted into a left- or right-circularly-polarized field, controlled via the optical power. The observations are explained by Kerr-nonlinearity induced symmetry breaking, which splits the resonance frequencies of degenerate modes with opposite polarization handedness in an otherwise symmetric resonator. The all-optical polarization control is demonstrated at threshold powers down to 7 mW. The physical principle of such Kerr effect-based polarization controllers is generic to high-Q Kerr-nonlinear resonators and could also be implemented in photonic integrated circuits. Beyond polarization control, the spontaneous symmetry breaking of polarization states could be used for polarization filters or highly sensitive polarization sensors when operating close to the symmetry-breaking point.

Octave spanning tunable frequency comb from a microresonator.

We report the generation of an octave-spanning optical frequency comb in a continuous wave laser pumped microresonator. The generated comb spectrum covers the wavelength range from 990 to 2170 nm without relying on additional external broadening. Continuous tunability of the generated frequency comb over more than an entire free spectral range is demonstrated. Moreover, the linewidth of individual optical comb components and its relation to the pump laser phase noise is studied. The ability to derive octave-spanning spectra from microresonator comb generators represents a key step towards f-2f self-referencing of microresonator-based optical frequency combs.

Photonic chip-based optical frequency comb using soliton Cherenkov radiation.

Optical solitons are propagating pulses of light that retain their shape because nonlinearity and dispersion balance each other. In the presence of higher-order dispersion, optical solitons can emit dispersive waves via the process of soliton Cherenkov radiation. This process underlies supercontinuum generation and is of critical importance in frequency metrology. Using a continuous wave-pumped, dispersion-engineered, integrated silicon nitride microresonator, we generated continuously circulating temporal dissipative Kerr solitons. The presence of higher-order dispersion led to the emission of red-shifted soliton Cherenkov radiation. The output corresponds to a fully coherent optical frequency comb that spans two-thirds of an octave and whose phase we were able to stabilize to the sub-Hertz level. By preserving coherence over a broad spectral bandwidth, our device offers the opportunity to develop compact on-chip frequency combs for frequency metrology or spectroscopy.

Mucociliary clearance and buffered hypertonic saline solution.

Nasal irrigations have been used for centuries without any scientific data to determine efficacy. For 10 years, the senior author has used buffered hypertonic saline nasal irrigation for patients with acute/chronic sinusitis and for those having undergone sinus surgery. A simple study was undertaken using volunteers without any significant sinonasal disease. Patients served as their own control using a saccharin clearance test before any nasal irrigation was used. Patients then used one of two solutions to irrigate their nose-buffered normal saline or buffered hypertonic saline-and were then retested. On a separate day, the control test was repeated, followed by irrigation with the alternate solution and a second saccharin clearance test. The outcome showed buffered hypertonic saline nasal irrigation to improve mucociliary transit times of saccharin, while buffered normal saline had no such effect.

In vitro testing in pediatric food allergy.

Food allergies are an important cause of morbidity in the pediatric population. We used a questionnaire to assess the value of in vitro testing in evaluating children for food allergy. The patient population was divided into two groups: those who tested positive (Class I or greater) for food allergy (combined food/inhalant allergy or food allergy alone) and those who tested negative for food allergy (inhalant only or negative test). Although we recognize that a significant proportion of food allergies may not be IgE-mediated, for the purposes of this study we considered a patient food allergy--positive if in vitro testing was positive and food allergy--negative if testing was negative. Thirty-eight of 77 patients tested (49%) were positive for food allergies. Those with food allergies did not have pretreatment scores significantly different from those of patients negative for food allergies (55 vs. 65, p = 0.11). Similarly, we detected no statistically significant difference between the symptom-improvement scores of the two groups (37 vs. 40, p = 0.57). We conclude that in vitro testing for food allergies is warranted in children with allergic symptoms, for several reasons: The prevalence of IgE-mediated food allergy in children may be higher than previously thought, and children with food allergies experience the same severity of symptoms as those without. It must be remembered that in vitro testing only detects IgE levels, and therefore a negative test does not mean that the patient does not have food allergies. However, when positive, in vitro testing gives the physician the opportunity to modify the treatment plan, helping avoid time-consuming and demanding oral food challenges. Our questionnaire also showed that food allergy and non-food allergy patients had similar pretreatment and posttreatment symptom scores.

Diagnosing and treating pediatric allergy.

Allergy is a significant problem in as much as 60% of patients seeking care at an otolaryngologist's office. This raises the issue of the most effective means of diagnosing allergy, especially in pediatric patients, who often may not tolerate skin testing. The RAST and CAP (Pharmacia) tests have been shown to be quite comparable with skin testing in diagnosing allergy. This study was undertaken to assess the effectiveness of treatment of children with allergies who had undergone in vitro testing. This was done by obtaining subjective symptom scores both before and after treatment. These were obtained from patients with different in vitro results, including some with negative testing. Overall, 77 of 155 mailed questionnaires were returned. Results showed overall improvement of 38.81 points in 12 symptoms (p = 0.001; n = 62). Each symptom was rated on a visual scale of 1 to 10. A small portion of the patient population received surgical treatment (tympanostomy tube placement with or without adenoidectomy) in addition to medical treatment and avoidance. These patients did not have significantly higher pretreatment scores and did not have a higher improvement rate for all symptoms. We believe this study shows the validity of in vitro testing and treatment of allergy in children.

Delayed diagnosis of a laryngotracheoesophageal cleft.

Laryngotracheoesophageal (LTE) clefts are rare anomalies of the airway. Surgical correction has been performed in all cases reported in the literature for clefts diagnosed during life. Posterior laryngeal clefts (PLC) are defined as minor (type I) to severe (type IV), with types III and IV being further described as LTE clefts. Type I clefts have been observed with an increased frequency of up to 6% of all pediatric direct laryngoscopies performed at a tertiary pediatric referral hospital. These can be managed medically and seldom require surgical correction. A case is presented of a boy whose initial diagnosis of LTE cleft (PLC type III) was made at age 7 years. His presenting complaints were chronic airway symptoms without airway distress. He developed only two episodes of pneumonia. Surgical correction was not performed. This young athlete has been followed for more than 6 years without significant problems. This appears to be the only case found in the literature of the successful management of a LTE cleft that has not required surgical correction. Photographic documentation and suggestions on endoscopic diagnosis are presented.

Family planning for inner-city adolescent males: pilot study.

The results of a pilot family planning program (FPP) in a pediatric practice are reported for 66 inner-city male adolescents. The FPP was designed to eliminate registration barriers to the procurement of contraceptives by adolescent males, and to prompt the community neighborhood health center (CNHC) providers to initiate discussion of birth control with their male adolescent patients. Utilization data show that males were more likely to receive contraceptives if the provider first raised the topic of birth control to them. Unstructured follow-up interviews with 27 adolescent male FPP users identified a desire for anonymity/confidentiality and embarrassment or discomfort as the key reasons for not seeking contraceptives from the CNHC. If health care providers are sensitive to male adolescents' feelings about birth control, community health centers potentially could serve as a cost-effective source of contraceptives for sexually active male adolescents.

PIP: The results of a pilot family planning program (FPP) in a pediatric practice are reported for 66 inner-city US male adolescents. The FPP was designed to eliminate registration barriers to the procurement of contraceptives by adolescent males, and to prompt the community neighborhood health center (CNHC) providers to initiate discussion of birth control with their male adolescent patients. Utilization data show that males were more likely to receive contraceptives if the provider first raised the topic of birth control to them. Unstructured follow-up interviews with 27 adolescent male FPP users identified a desire for anonymity and confidentiality, and embarrassment or discomfort as the key reasons for not seeking contraceptives from the CNHC. If health care providers are sensitive to male adolescents' feelings about birth control, community health centers potentially could serve as a cost-effective source of contraceptives for sexually active male adolescents.

Mid-infrared optical frequency combs at 2.5 µm based on crystalline microresonators.

The mid-infrared spectral range (λ~2-20 µm) is of particular importance as many molecules exhibit strong vibrational fingerprints in this region. Optical frequency combs--broadband optical sources consisting of equally spaced and mutually coherent sharp lines--are creating new opportunities for advanced spectroscopy. Here we demonstrate a novel approach to create mid-infrared optical frequency combs via four-wave mixing in a continuous-wave pumped ultra-high Q crystalline microresonator made of magnesium fluoride. Careful choice of the resonator material and design made it possible to generate a broadband, low-phase noise Kerr comb at λ=2.5 µm spanning 200 nm (≈10 THz) with a line spacing of 100 GHz. With its distinguishing features of compactness, efficient conversion, large mode spacing and high power per comb line, this novel frequency comb source holds promise for new approaches to molecular spectroscopy and is suitable to be extended further into the mid-infrared.