Fifteen years ago, when Koji Kitamura opened Ajihei, his small, below-stairs sushi restaurant on Chambers Street, it quickly gained a reputation for pristine seafood, expertly made sushi rice, and this chef/owner’s prodigious knife skills.

Ever since, it has been considered Princeton’s premier, albeit quirky, sushi spot, securing even statewide recognition. (It currently, for example, boasts a food score of 24—excellent—in the N.J. Zagat Survey.)

Ajihei’s menu, which has always included Japanese appetizers and hot dishes as well as sushi and sashimi, didn’t change much over the ensuing years so regulars were surprised when, after temporarily closing in mid-2014 to revamp the space, Kitamura reopened with a streamlined carte that greatly condenses the number of sushi and sashimi offerings and features ramen—three kinds—for the first time.

“Every cook in Japan knows how to make ramen,” Kitamura, who is 54 and works seven days a week, told me on a recent visit. He curtailed his sushi (and other) offerings to make life easier on himself, so for the first time he hired a younger sushi chef to help out on weekends. (The restaurant is closed on Mondays.)

Kitamura says he added ramen not because it’s trendy, but in anticipation of perhaps someday opening a restaurant in Manhattan or Brooklyn, where ramen would be expected to be on the menu. Ajihei offers three kinds: shio, for which the main seasoning source is sea salt; shoyu, featuring soy sauce in that role; and 10-hour tonkotsu, which refers to the dish’s broth, made from pork bones.

That last is further defined as “paitan soup,” which means “white” or “milky” in Japanese and that does, indeed, describe Kitamura’s broth. Even though the menu says 10 hours, he says it can take up to 18 hours to make to his satisfaction, depending on the bones.

He proudly showed this reporter into his restaurant’s kitchen to see a big pot of Flintstone-size bones, stripped of their meat, bubbling away. Stacks and stacks of more such bones awaited a similar fate. He also handed me, with great disdain, a webpage printout of bottled ramen base concentrates — shoyu, miso and tonkotsu — that he says many restaurants buy and dilute to make their broth.

A generous bowlful of Ajihei’s tonkotsu ramen ($12 at lunch) comes with ramen noodles that are thin, straight and white (as opposed to the curly, bright yellow ones made famous by Cup Noodles), along with traditional toppings. These include chasu, a slab of boneless pork (usually shoulder), scallions, a square of nori and bamboo shoots. Thin strands of bright pink pickled ginger add zest, and patrons can customize their ramen with chili oil, bottles of which stand at the ready on the restaurant’s blond wood tables and counters, as does soy sauce.

The broth for the other ramen types, shoyu and shio, can be made from any combination of chicken, pork or fish. At Ajihei, a bowl of either is $10 at lunch, sports the same toppings and includes a dumpling of the day. Tellingly, Kitamura being the traditionalist he is does not offer miso ramen, which is a relatively new addition to the ramen pantheon, first making an appearance in Japan in the 1960s.

He adheres to Japanese ramen tradition in another way, too: he refuses to offer it to go. “One young lady who wanted it to go yelled at me and made quite a fuss because I wouldn’t sell it to her!” he recalls. Serious ramen chefs maintain that once the noodles are added to the broth, they will become mushy within five minutes. (Some shops that allow takeout keep the noodles and broth separate.)

In addition to offering ramen and bringing in a second chef, Kitamura made one other change that some of his longtime regulars bemoan. It used to be that diners could show up and request “chef’s choice” sushi and sashimi any night. Now, omakase can be had only with prior notice of two to three days. As a reward, Kitamura offers an ever-changing array of off-menu specialties, made with ingredients like toro (fatty) tuna, Wagyu beef, and fish heads, innards or other uncommon cuts.

Ajihei earned the “quirky” label because of the strict dining rules that Kitamura laid down from the start and has stuck to ever since, chief among them that he will not seat parties of more than four. His explanation, then as now, is that (a) the quality of his made-to-order creations would suffer by the time one large table’s were ready for simultaneous delivery, (b) service to the remaining tables would back up, and (c) the restaurant seats only about two dozen guests, so accommodating one or two large parties would mean that many more smaller ones would have to wait a long time for a table.

Koji Kitamura lives in West Windsor with his wife, Kaoru, who runs the restaurant with him. They have two college-age children. Kitamura hails from Ogaki, in Japan’s Gifu prefecture, where his parents still live. Ogaki has the distinction of being known as the most centrally located city in that country. His father worked for the Ogaki city hall, and his mother taught elementary school and music. Back in 2005, Gifu was the site of the World Rowing Championships and when Anne and George Carcagno of Pennington, rowing enthusiasts and Ajihei regulars, told Kitamura they would be attending, he arranged for them to visit his hometown. As a memento, they presented Kitamura with a framed photo of Ogaki’s castle.

Kitamura left Japan for the U.S. a few years after graduating from college, because at that time the cost of living in Japan was prohibitive and relatively cheap in the U.S. During college he had worked in a restaurant to pay for his living expenses, and eventually found work in New York with a private sushi caterer. As tradition holds, he had to pay $5,000 to be trained by the sushi master there, but he earned that and more back from the same fellow within a year, once he was hired there to make sushi. For a short time Kitamura worked on the line at Nobu, the famed New York restaurant of Masaharu Morimoto. When he decided he wanted to open his own restaurant, Kitamura found the rents in New York too high, which led him to Princeton, where he thought the international community would appreciate his talents.

The limit on party size is just one of several rules at Ajihei. Only one bottle of wine for each two guests is allowed at this b.y.o.b., and hard liquor not at all. If you do bring your own wine, the minimum food charge per person is $20 (which, in all honesty, is easy to reach). From opening time to 7:30 pm, parties are expected to enjoy their meal and then vacate their table within 90 minutes. No birthday cakes are allowed in. The reason for just about all of these rules? So that Kitamura can accommodate the maximum number of diners who want to enjoy his specialties, and to focus on them without distractions. His is too small a restaurant, Kitamura says, for lingering of an evening. Still, he considers himself flexible. “In Japan,” he points out, “serious ramen shops don’t allow conversation of any kind.”